MEDICAL HINTS (by the late William Henry Cross, M.D. Revised
by Andrew Balfour, C.B., G.M.G., M.D.) 160-292
Introduction, 160—General Hints, 168—Diseases and their
Prevention and Treatment, 169—Medicines, Medical
Appliances, &c., 252—Treatment of Wounds and Injuries,
275.
CANOEING AND BOATING (by the late J. Coles) 293-297
ORTHOGRAPHY OF GEOGRAPHICAL NAMES (by Maj.-General Lord
Edward Gleichen, K.C.V.O.) 298-305
ON THE GIVING OF NAMES TO NEWLY-DISCOVERED PLACES 306
INDEX 307-318
MAPS AND ILLUSTRATIONS.
PAGE
Diagram showing how to read Thermometer Scale 13
Mr. H. F. Blanford’s Portable Thermometer Screen 15
Hut for Sheltering Thermometers 16
Section of Assmann’s Aspiration Psychrometer 20
Diagram showing how to read Barometer Vernier 27
The Collie Barometer, with the Deasy Mounting 30
Diagram showing Cyclone Paths and Circulation of Winds in Cyclones 33
Charts of the World showing Isothermal Lines for January and July 50
Charts of the World showing Isobaric Lines for January and July 50
Rainfall Chart of the World 50
Bellows Camera 52
Twin-Lens Camera 53
Homocentric Lens 55
General Collecting Case for Natural History Specimens 93
Drying Press for Botanical Specimens 100-101
Diagrams of the Human Skull illustrating Craniological Descriptions 134
Diagrams of the Human Skull illustrating Cranial Measurements 135
Diagram of Portion of the Thigh-Bone for Measurement of its length 137
Diagram showing some of the Principal Organs of the Body, and the
Course of the Main Blood-Vessels 162
Spectacles for Preventing Snow-Blindness 199
Diagram illustrating method of compressing the Main Artery of
the Thigh 278
Diagrams illustrating methods of Restoring Breathing in cases
of Drowning 284
Diagram of the Human Skeleton, giving the Names and Positions
of the Chief Bones 285
Diagrams showing Bandaging of Broken Jaw 289
HINTS TO TRAVELLERS.
_VOL. II._
I.
METEOROLOGY AND CLIMATOLOGY.
_By HUGH ROBERT MILL, D.SC, LL.D., F.R.S.E., Formerly President
Royal Meteorological Society, and Director of the British Rainfall
Organization._
The nature of the meteorological observations made by a traveller or by
a resident in regions where there is no organised meteorological service
will necessarily depend on the object which he has in view, the time he
is able to devote to meteorological work, his knowledge of meteorology as
a science, and his interest in it.
Of the many ways in which a traveller may add to the knowledge of
atmospheric conditions, five may be specially mentioned:—
1. _A record of the weather, observed day by day with regard
both to non-instrumental observations and the readings of
instruments._ This may be taken as the minimum incumbent on all
travellers.
2. _Observations for forecasting the weather and obtaining
warning of storms._ This is sometimes of vital importance; it
is always valuable at the time, and occasionally the results
are worth recording. It may, however be looked upon as a
practical application of the systematic observations.
3. _Observations with a view to determining the character of
the local climate._ The traveller passing through a country can
do little in this way, as long continued uniform observations
in one place are necessary to fix the annual variations.
Still, the recording of such data as may be obtained is always
important in a little-known region, and the work of several
travellers at different seasons will allow some fair deductions
to be drawn. When a day is spent in camp, much importance
attaches to regular observations made every two hours, from
which the diurnal changes of climate may be ascertained.
4. _Special meteorological researches._ These usually demand
special instruments and skilled observers. Observations in the
upper air by kites or balloons in particular, must be an end in
themselves. Exact measures of radiation in deserts, of rainfall
in forests and on adjacent open ground, of temperature during
land and sea breezes, or of fogs, thunderstorms, tornadoes,
etc., in places subject to those visitations, are always of
value. As a rule, however, the traveller cannot devote much
time to these matters, unless the study of physical geography
is the object of his journey.
5. _The collection of existing meteorological records._ It
sometimes happens that at outlying stations meteorological
observations have been taken and recorded for a considerable
time. If they have not been already communicated to some
meteorological centre, the traveller should obtain a copy of
them, and also compare the instruments in use with his own.
He might in some cases aid in securing a knowledge of local
climate by inducing residents at outlying stations to start
regular observations.
The first two ways of advancing meteorology need alone be considered in
detail; but with regard to all, it must be clearly understood that the
value of the work is greater the more carefully the observations are made
and recorded, and the more remote and less known the region.
1. A RECORD OF WEATHER.—The traveller who makes his journey for any
other purpose than the study of physical geography would be wise to
burden himself as little as possible with instruments, but to understand
thoroughly and use faithfully the few he carries. In a rapid march many
different climates may be traversed in a few weeks, and the records of
variation of weather so obtained could not have much value; but when a
halt of a few days or of a week or two is made, systematic observations
become valuable.
NON-INSTRUMENTAL OBSERVATIONS.
The first place must be given to non-instrumental observations, which may
be made at any time on the march or in camp, and should always be noted
at the time they are made in the rough note-book, and copied carefully
into the journal each evening. These observations in the rough note-book
will necessarily be mixed up with others on various subjects; but the
meteorological facts should have a place reserved for themselves in the
journal, say at the end of each day’s work.
_Wind._—Observations of the direction and force of the wind at several
fixed hours in the day are advisable for comparison with instrumental
readings; but on the march every decided change should be recorded if
the nature of the country permits. In the depths of a forest, or in a
narrow valley, the wind, if felt at all by the traveller, gives scarcely
any clue to the movement of the air over the open country, but in most
cases the movements of low clouds, when any are in sight, may be taken
as a fairly satisfactory test. The direction is to be observed by means
of the compass, and it will be sufficient to estimate it by the eight
principal points—North, North-east, East, South-east, South, South-west,
West, and North-west. Any sudden changes in direction so pronounced as
to be noticeable should be recorded, for, taken in conjunction with the
barometer readings, if the journey is along a route of nearly constant
level, they are valuable aids in predicting the weather. In some places
the direction of the wind undergoes a well-marked regular diurnal change
in perfectly settled weather.
Wind is always named by the direction from which it blows. The force of
the wind is best estimated on the scale Calm, Light, Moderate, Fresh,
Strong, and Gale. It is impossible, without long experience and the
tuition of a trained observer, to assign relative numbers to these
forces which should have any permanent value for comparison with the
observations of others. Travelling on foot in a strong wind is always
uncomfortable, and in a gale very difficult. If it is impossible to
make way against the wind at all, or to pitch tents, the force may be
put down as Hurricane after it has passed, the traveller bearing in
mind that if he can write in his note-book at all, while unsheltered,
a hurricane is not blowing. If a lake or a river without appreciable
current is in sight, wind just sufficient to produce white crests on the
waves may be called fresh, and that sufficient to blow away spray from
the crests deserves to be termed strong. At sea, in a sailing-vessel,
it is possible to acquire great skill in estimating wind-force; hence
Beaufort’s scale, originally devised with reference to the amount of sail
a well-equipped frigate could carry, has come into extensive use, and it
is as well to know it. By comparison with anemometers, the approximate
velocity in miles per hour corresponding to the numbers on the scale has
been estimated:—
BEAUFORT’S SCALE OF WIND FORCE.[1]
-----+-----------------+------------------
| | Mean Velocity in
No. | Name. | miles per hour.
-----+-----------------+------------------
0 | Calm | 0
1 | Light air | 1
2 | Light breeze | 4
3 | Gentle breeze | 9
4 | Moderate breeze | 14
5 | Fresh breeze | 20
6 | Strong breeze | 26
7 | Moderate gale | 33
8 | Fresh gale | 42
9 | Strong gale | 51
10 | Whole gale | 62
11 | Storm | 75
12 | Hurricane | 92
-----+-----------------+-----------------
The duration of strong wind should be noted, as well as the time of any
marked change of strength. The land and sea winds of tropical coasts show
a well-marked relation to the position of the sun and the hour of sunset,
and in places where these winds blow the hours of calm and change should
be noted. On mountain slopes a similar diurnal effect may be noticed;
the wind usually blows uphill during the day and downhill at night,
while in valleys it usually blows either with or against the direction of
the river. Local winds of peculiar character are sometimes met with in
association with mountains such as the Föhn of the Alps, the Chinook wind
of the Rocky Mountains, and the Helm wind of the Eden Valley in England
and Adam’s Peak in Ceylon.
Whirlwinds and tornadoes are rare phenomena, but if met with, it is worth
while to take some trouble to put on record at least the hour of their
appearance (local time), the direction in which the whirl moves onward,
and the breadth of the path of destruction it leaves behind. When a storm
of wind has passed over a wooded region and blown down many trees, the
direction in which most of the trunks lie is worth observing. The top of
the tree usually falls in the direction in which the wind was blowing,
hence the root usually points to the direction of the wind. Waterspouts
are closely allied to whirlwinds, and in any of those phenomena of
revolving columns of air it is of much theoretical importance to
determine the direction of the whirl about the axis, _i.e._, whether the
rotation is in the direction of the hands of a watch or the opposite.
The prevailing wind of a district may often be discovered by the slope
of trees growing on open ground, or still better, by the difference in
the degree of wave erosion on small lakes. If the banks are of the same
material all round, the side against which the prevailing wind drives the
waves will always be the most worn away.
_Cloud and Sunshine._—It would be impossible to keep a record of the
countless changes in the cloud-covering of an English sky, but in many
parts of the world the absence or presence of cloud is a function of
latitude, altitude, and season, of great stability, and worthy of being
attentively studied. The amount of cloud is usually estimated as the
number of tenths of the sky covered; but it is a very difficult thing
indeed to compare a tenth of the visible sky near the horizon with a
tenth near the zenith. There is no difficulty, however, in observing when
the sky is completely overcast or quite free of cloud, and as a matter of
convenience the belt round the horizon to the height of thirty degrees
may be neglected, _i.e._, the lower third of the distance from the
horizon to the zenith. Very often it will be found that clouds form and
disappear at certain hours of the morning or evening, and it is useful to
get exact information on the subject.
Of more importance than the amount of cloud is its nature, elevation,
and movement. Distinct species of cloud have been recognised for a long
time, and from more recent studies it would appear that they owe their
distinctive appearance to the altitude at which they float in the air.
Meteorologists distinguish a number of classes and transitional forms of
cloud; it is enough for the traveller to be able to recognise the most
definite types, viz., Cirrus, Cumulus, Stratus, and Nimbus. _Cirrus_
clouds are the small tufts or wisps of cloud which float very high in the
atmosphere, and to which the popular name of “mare’s tails” is applied.
The transitional form, _Cirro-Cumulus_, popularly known as “mackerel
scales” or “mackerel sky,” is equally easy to identify. _Cumulus_ clouds
are great woolly-looking heaps of cloud, the lower surface of which
is often nearly horizontal, while above they well into an exuberant
variety of rounded forms. They represent the condensation of moisture
in ascending columns of heated air. _Stratus_ clouds are low-lying
sheets of condensed moisture, which, being usually seen at a low angle,
appear like thin layers parallel to the horizon. The transitional type
_Cirro-Stratus_ is usually seen in the form of great feather-like clouds
stretching across nearly the whole sky. _Nimbus_ is a rather low-lying
cloud from which rain is falling even if the rain is re-evaporated
before reaching the ground. The lowest clouds of all, those resting
on the surface of the ground and enveloping the observer, are called
_mist_ and _fog_. The two are distinguished by the fact that a mist wets
objects immersed in it, while a fog does not. All clouds, except Cirrus,
are physically the same, consisting of minute globules of liquid water
falling through a portion of air saturated with moisture. The globules
being small offer a relatively great surface to friction, and so fall
very slowly, and in the higher clouds they evaporate on the lower surface
before they have time to agglomerate into raindrops. In the highest of
all clouds, the cirrus type, the particles are spicules of ice and not
globules of water. It is a common error to suppose that black clouds
differ from white clouds. All clouds are white when they reflect the
light of the sun, and all are black when they come between the eye and
the sun in sufficient thickness to cut off a considerable portion of its
light.
The sudden appearance of a particular kind of cloud is important as
a weather sign. It shows that changes are going on in the vertical
circulation of the atmosphere. Hence if cirrus or cumulus cloud should be
observed to be increasing the fact should be noted, and the direction in
which the clouds are moving should be noted also.
In observing cloud-motion attention should be given only to the sky
overhead; at any lower angle the parallax due to viewing the clouds
obliquely deprives the observation of value. It is also necessary to
distinguish between the movement of the upper and of the lower clouds,
as these are floating in very different parts of the atmosphere. It is
comparatively rarely that the motion, say of nimbus and cirrus, is in the
same direction. On a lofty mountain, strata of cloud which from below
were seen to be cumulus may be passed through as layers of mist, and on
emerging from them their upper surface may be seen below one. In many
mountains the cloud-belt is as sharply defined as the snow-line, and its
variations should be carefully observed.
Clouds should occasionally be photographed as a record. This should
be done especially when a type of cloud comes to be recognised as a
usual one, for while exceptional forms may prove interesting, a record
of the usual forms is certain to be valuable. In this connection a
protest may be made against the horrible custom of some amateur and of
many professional photographers of printing in clouds from some stock
negative in their pictures of scenery. The cloud is an essential part of
a picture, and it is better to leave an over-exposed sky of natural cloud
than to insert a beautiful representation of a cloud-form which may be
one never visible in the particular place or at the particular season.
_Mist, Fog and Haze._—Mist or fog at low levels will of course be
recorded whenever observed, and its density and duration noted. A good
way to define the density of thick fog is to measure the number of yards
at which an object becomes indistinguishable, and the most convenient
object for the purpose is a person. Light mists lie over water or marshes
at certain hours in particular seasons, and their behaviour should always
be observed. It often happens that the distant view from a height is
obscured by a haze not due to moisture, and this appearance should be
noticed with a view to discovering its cause. The smoke from forest or
prairie fires in Canada sometimes produces so thick a haze as to put a
stop to surveying operations for weeks at a time. Haze is often due to
dust blown from deserts, or ejected from volcanoes, and sometimes to
swarms of insects.
_Rain and Dew._—The journals of most travellers fail to give a clear
idea of the prevalence of rain during their journeys, and it is much to
be desired that something more explicit than “a showery day” or “fairly
dry” should be recorded. The hour of commencement and cessation of rain
during a march should be noted, and some indication given as to whether
the rain fell heavily or lightly. In this way any tendency to a diurnal
periodicity of rain would be detected, and some definite meaning would be
given to the terms rainy season and dry season. If rain occurs during the
night it should also be recorded, and the amount of night rains should
always be measured by means of a rain gauge in the manner to be described
later.
The general condition of a country with regard to rain may often be
judged from the appearance of vegetation or the marks of former levels
of high-water in lakes or rivers. Thus on mountain slopes or the sides
of a valley any difference in the luxuriance of vegetation according to
exposure probably indicates the influence of rainfall as guided by the
prevailing wind. So, too, the appearance of lines of drifted débris on
the banks some distance from the edge of a lake or river may be taken
as indications of the height to which the waters sometimes rise; and
conversely the appearance of rows of trees in the middle of a wide
shallow lake may indicate the line of a river which has temporarily
flooded the surrounding meadows. Such observations have an important
bearing on climate.
The appearance and amount of dew are also to be recorded. The most
important points to notice are the hour in the evening when the deposit
commences, and the hour in the morning when the dew disappears. It should
be noted also whether the deposit of dew is in the form of small globules
standing apart on exposed surfaces, or if it is heavy enough to run
together into drops and drip from vegetation to the ground.
_Thunderstorms and Hail._—The occurrence of thunderstorms should of
course be noted, and here the hour of occurrence is of very great
importance, for thunderstorms frequently show a marked diurnal period.
The appearance of lightning without thunder should be recorded when it
is observed, but this will naturally be almost always after sunset.
Hailstorms usually accompany thunderstorms, and sometimes take the place
of them. The occurrence of hail is most frequent in summer, and records
of the size of hailstones are important. If possible they should, when
very large, be photographed along with some object of known size, and
their structure described. It might at least be noticed whether they are
hard and clear, like pure ice, or opaque like compacted snow, or made up
of concentric layers of clear and opaque ice alternately.
_Snow._—Snow falls in all parts of the world, although in tropical or
sub-tropical latitudes only at great elevations above sea-level. The
actual limits of snowfall at sea-level are as yet imperfectly known, and
any observations of snow showers in the neighbourhood of the tropics
are of importance. It is essential in such a case to record also the
approximate elevation of the land. On mountains in all latitudes the
position of the snow-line should be noted at every opportunity. This is
the line above which snow lies permanently all the year round, or below
which snow completely melts in summer; and it is a climatic factor of
some importance. It may be remarked, for instance, that if the traveller
finds snow lying on grass, moss, or other vegetation, he is certainly
not above the snow-line. It is necessary also to notice that glaciers
may descend unmelted a long distance below the level of perpetual snow.
While the conditions of snow lying on the ground in the Arctic regions
and above the snow-line in any part of the world are matters pertaining
more to geology and mountaineering than to meteorology, the duration of
snow-showers, the character of the snow, and the depth to which it lies
on ground below the snow-line are too important from their bearing on
climatology to be overlooked.
The character of the snow as it falls varies from the sleety, half-melted
drops common in warm air to the fine dust of hard, separate ice-crystals
found in the intense cold of a Polar or Continental winter. The feathery
appearance of lightly-felted flakes is an intermediate type between the
two extremes. In measuring the depth of snow as it lies, care should be
taken to select open ground where there is no drifting, and when the
snow is not too deep the measurement can usually be best made with a
walking-stick on which a scale of feet and inches (or of centimetres)
has been cut. Such a stick is useful for measuring the depth of shallow
streams, and for many other purposes. The result should be entered as
“depth of fallen snow,” so that there may be no risk of confusing the
figures with the amount of snowfall estimated as rain. Speaking roughly,
a foot of snow is usually held to represent an inch of rain. A violent
storm of wind, accompanied with falling or driving snow, is termed a
_blizzard_ in the western United States, and a _buran_ in Siberia. The
name blizzard has been naturalized in the Antarctic, but it is not known
that this phenomenon is identical with the American storm.
_Frost._—The appearance of frost in the form of hoar-frost (the way
in which atmospheric water-vapour is deposited in air below the
freezing-point), or of thin ice formed on exposed water, should always
be carefully looked for and noted. In hot, dry countries the intense
radiation from the ground at night often reduces the temperature below
the freezing-point, although, during the day, the ground may be very hot.
The appearance of frost at sunrise is a valuable check on the readings
of a minimum thermometer, and in most cases is a more trustworthy datum.
Similarly in cold countries, where snow is lying on the ground or ice
covering the rivers, the appearance of thaw, especially in cloudy
weather, is a delicate test of the rise of the air temperature to the
freezing-point. The traveller should never fail to record cases of
melting and solidifying of any substances due to changes of temperature.
The softening of candles and the freezing of mercury or of spirits give
information regarding temperature at least as valuable as the readings of
thermometers.
_Other Observations._—Any peculiar atmospheric phenomena, such as the
appearance of the zodiacal light after sunset, the aurora, the electrical
lights seen on pointed objects, and known as St. Elmo’s fire, rainbows,
especially lunar rainbows, haloes, the appearance of mock-suns or moons,
meteors or shooting stars, should be noted on their occurrence, as many
of them are valuable weather prognostics. Attention should also be given
to any appearances of mirage, or other effects of irregular distribution
of atmospheric density. A mirage is only rarely so perfect as to show
ships inverted in the air, palm-grown islands in the sea, or distant
oases in the desert. The common form is an unusual intensification of
refraction, raising land below the horizon into sight, or apparently
cutting off the edges of headlands or islands at sea or on large lakes.
It is worth while observing the temperature of the air and of the water
or ground when an unusually clear mirage effect is visible.
Another interesting series of observations may be made on the colours of
the sky and clouds at sunrise and sunset. A phenomenon often observed at
sunset, but the existence of which is still sometimes denied, should be
looked for. This is the appearance of a gleam of coloured light at the
moment when the upper edge of the sun dips below a cloudless horizon. A
note should be made of the nature of the horizon, whether land or sea,
and of the colour of the light if it should be observed. When opportunity
offers, the first ray of the rising sun might be similarly observed.
The traveller should, at the end of each day, give his opinion of the
nature of the weather, saying whether he felt it hot or cold, relaxing
or bracing, close or fresh. Such observations have no necessary relation
to degree of temperature or humidity as recorded by instruments; but
the human body is the most important of all instruments, and everything
which affects it should be studied. By paying attention to the foregoing
instructions, an observant traveller will bring home a far better
meteorological log without instruments than a more careless person
could produce by the diligent reading of many scales. Yet, in enforcing
the importance of non-instrumental observations, we must not leave the
impression that the readings of instruments are of little value. It is,
in ordinary circumstances, only by the readings of instruments that the
climate of one place can be compared with that of another, and only the
best results of instrumental work are precise enough to form a basis for
climatological maps.
INSTRUMENTAL OBSERVATIONS.
The minimum requirement of instrumental observations by a traveller is
the reading twice daily of the barometer and of the dry and wet bulb
thermometers, to ascertain the temperature and humidity of the air,
also the reading once daily, in the morning, of the minimum thermometer
which has been exposed all night, and on days in camp of the maximum
thermometer also. It is very desirable to expose a rain gauge whenever
it is practicable to do so. Unless special meteorological researches are
to be carried out, nothing farther in the way of observations need be
attempted. A very useful supplement to the necessary observations is the
use of a self-recording barograph or thermograph; but these are delicate
instruments, liable to get out of order unless very carefully handled,
and it will not always be possible to make use of them.
The observer must understand what his instruments are intended to
measure, how they act, and how they should be exposed, read, and the
reading recorded. He must know enough about all these things to be
able to dispense with unnecessary precautions only possible at fixed
observatories, and, at the same time, to neglect nothing that is
necessary to secure accuracy in the results.
_Thermometer Corrections._—All thermometers, without exception, should
have the degree marks engraved on the stem, or on a slip of enamel within
the outer tube, and be supplied with a certificate from the National
Physical Laboratory showing the error of the scale at different points.
This certificate should be in duplicate, and a copy ought to be left
in a safe place at home. After a long journey the thermometers which
have been in use should be sent to have their errors re-determined. The
corrections are not, however, to be applied by the observer unless he is
working out his observations for some special purpose. No thermometer is
passed at the National Physical Laboratory if its error approaches one
degree, so that for all ordinary purposes of description a certificated
thermometer may be looked on as correct. But when the readings are being
critically discussed and compared with the observations of other people,
the correction is of the greatest importance. _It cannot be too strongly
impressed upon an observer that, in reading meteorological instruments,
he must read exactly what they mark, and record that figure in his
observation-book on the spot._ The corrections can be applied afterwards
by the specialist who discusses the work. For subsequent reference it is
necessary to note in the observation-book the registered N.P.L. number
of the thermometer in use, and if a thermometer should get broken and
another be used instead, the number of the new instrument must be noted
at the date where it is first employed. Care should be taken to use the
same thermometer for one purpose all the time if possible, and only an
accident to the instrument should necessitate a change being made.
Thermometers are either direct-reading or self-registering. The former
are used for obtaining the temperature at any given moment, the latter
for ascertaining the highest or the lowest temperature in a certain
interval of time. They are filled either with mercury, or a light fluid
which freezes less readily, such as alcohol or creosote.
_Thermometer Scales._—The particular system on which the thermometers are
graduated is of no importance, but merely a matter of convenience. The
Fahrenheit scale is used for meteorological purposes in English-speaking
countries; but for all other scientific purposes the Centigrade scale
is used everywhere. One can be translated into the other very simply
by calculation[2]; but it is convenient for a traveller to have all his
thermometers graduated in accordance with one scale only.
The graduation, as marked on the stem of the thermometer, is usually to
single degrees, but anyone can learn to read to tenths of a degree by
a little practice. Care must be taken to have the eye opposite the top
of the mercury column. Suppose it to be between 50 and 51, the exact
number of tenths above 50 is to be estimated thus: If the mercury is just
visible above the degree mark it is 50°.1, if distinctly above the mark
50°.2, if nearly one-third of the way to the next mark 50°.3, if almost
half-way 50°.4, exactly half-way 50°.5, a little more than half-way
50°.6, about two-thirds of the way 50°.7, if nearly up to the next mark
50°.8, and if just lower than the mark of 51° it is 50°.9. The eye soon
becomes accustomed to estimating these distances.
[Illustration: FIG. 1.—_Reading Thermometer Scale above and below Zero._]
In using a thermometer below zero, the observer must pay attention to
the change in the direction of reading the scale, the fractions of a
degree counting downward from the degree mark instead of upward from
it, as in readings above zero. Readings below the zero of the scale are
distinguished in recording them by prefixing the minus sign. The annexed
figure shows the reading of two thermometers graduated to fifths of
a degree, one showing a temperature of 1°.4, the other of -1°.4. The
British Meteorological Office now recommends the use of the Centigrade
thermometer graduated from the absolute zero, _i.e._, the freezing point
is shown by 273°, the boiling point as 373°.
_Care of Thermometers._—Mercurial thermometers will always be employed
for ordinary purposes in places where the temperature is not likely
to fall to -40°: _i.e._, everywhere except in the polar regions and
the interior of continents north of 50° N. These thermometers are very
strong and are not easily broken except by violence. The one vulnerable
part is the bulb, which is of thin glass and filled with heavy mercury.
Hence, in carrying thermometers, care has to be taken to protect the bulb
from coming in contact with any hard object. The best way to carry an
unmounted thermometer is in a closed brass or vulcanite tube with a screw
top, the inside of the tube being lined with india-rubber and provided
with a cushion of cotton-wool for the bulb to rest on. If the thermometer
is mounted in a wooden frame it should be secured in a box so that the
frame is firmly held and the bulb projects into a vacant part of the box,
which may be lightly filled with cotton-wool or provided with a deep and
well-padded recess. Every thermometer which is not graduated above 120°
should have an expansion at the top of the tube which the mercury that
may be driven beyond the scale by over-heating will not fill; otherwise
any accidental over-heating will break the bulb.
The unavoidable shaking or any sudden shock during travelling is apt
to cause the mercury column to separate, and a portion of it may be
driven to the top of the tube, where it may remain unless looked for and
brought back. Hence it is important to see that the top of the bore of
the tube is visible, and not covered by any attachment holding the tube
to a wooden frame. Thermometer readings are absolutely valueless unless
the whole of the mercury fills the bulb and forms a continuous column in
the stem. To bring a broken column together the best plan is to invert
the thermometer, if necessary shaking it gently, until the mercury flows
from the bulb and entirely fills the tube, leaving a little vacant dimple
in the mass of mercury in the bulb. When this is done, the thermometer
should be brought into its normal position bulb downwards, and the column
will usually be found to have united. If this method does not succeed the
thermometer may be held in the hand by the upper end, raised to the full
stretch of the arm, and swung downwards through a wide arc with a steady
sweep. I have never known this method to fail.
_Thermometer Screens._—It is usual at fixed stations to expose the
thermometer to the air by hanging it in a screen made of louvre-boards
so arranged that the air penetrates it freely while the direct rays of
the sun are cut off. The Stevenson screen, constructed on this plan with
a door opening on the side away from the sun, is well adapted for use
in temperate countries; but it is too cumbrous to carry on a journey and
does not afford sufficient ventilation for use in tropical countries.
An excellent substitute is the canvas screen devised by the late Mr. H.
F. Blanford, which consists of a bamboo frame carrying the thermometers
(with their bulbs four feet from the ground). The whole structure is five
feet high, and is sufficient for any places where the wind is moderate.
It is constructed of bamboos or rods of light wood, cords, and canvas,
which may readily be made up before starting, and it is easily renewed
or repaired. The canvas roof should be triple or quadruple according
to the thickness of the material. Such a screen will afford sufficient
protection at night, or even in the day, if set up in the shade, and it
will throw off rain; but in the sun it will require a thick mat as an
additional protection on or preferably stretched above the roof.
[Illustration: FIG. 2.—_Mr. H. F. Blanford’s Portable Thermometer
Screen._]
For a more permanent station the form of exposure recommended by a
committee of the British Association for use in tropical Africa will be
found very suitable in hot countries.
[Illustration: FIG. 3.—_Hut for Sheltering Thermometers._]
The thermometers are placed in a galvanised iron cage, which is kept
locked for safety. This cage is suspended under a thatched shelter,
which should be situated in an open spot at some distance from buildings.
It must be well ventilated, and protect the instruments from being
exposed to sunshine or rain, or to radiation from the ground. A simple
hut, made of materials available on the spot, would answer this purpose.
Such a hut is shown in the drawing (Fig. 2). A gabled roof with broad
eaves, the ridge of which runs from north to south, is fixed upon four
posts, standing four feet apart. Two additional posts may be introduced
to support the ends of the ridge beam. The roof at each end projects
about eighteen inches; in it are two ventilating holes. The tops of the
posts are connected by bars or rails, and on a cross bar is suspended the
cage with the instruments. These will then be at a height of six feet
above the ground. The gable-ends may be permanently covered in with mats
or louvre-work, not interfering with the free circulation of the air, or
the hut may be circular. The roof may be covered with palm-fronds, grass,
or any other material locally used by the natives for building. The floor
should not be bare but covered with grass or low shrubs.
The great object of these precautions is to obtain the true temperature
of the air, and avoid the excessive heating due to the direct rays or
reflected heat of the sun falling on the thermometers, and the excessive
cooling due to the radiation of heat from the thermometers to a clear
sky at night. Such a shelter is absolutely necessary when maximum and
minimum thermometers are used; but can be dispensed with for the simple
observation of the temperature of the air at a given time. This may be
effected by securing a rapid flow of air over the thermometer, either by
causing the air to flow past the instrument or by causing the instrument
to move rapidly through the air. It has been found by experiment that
the true temperature of the air is obtainable in this way whether the
operation is performed in sunshine or in shade; but it is preferable to
do so in the shade.
_Sling Thermometer._—The sling thermometer is the most simple and
convenient of all instruments for ascertaining the temperature of the
air. It is an unmounted thermometer with a cylindrical bulb, and the
degree-marks engraved on the glass stem. The upper end terminates in
a ring to which a silk cord about two feet long is attached. As a
precaution it is as well to secure the cord by a couple of clove hitches
round the top of the thermometer stem as well as to the ring, as the
thermometer would then be held securely even if the ring broke. The
thermometer is used by whirling it in a vertical circle about a dozen
times, the observer taking care, by having a loop of the string round the
wrist or finger, that it is not allowed to fly off. Then the thermometer
is read, swung once more, and read again. This process is repeated
until two consecutive readings are identical; when this is the case the
instrument shows the true temperature of the air. It is sufficient to
note the final temperature in the observing book.
The risk of breaking a sling thermometer is the only drawback to its use.
Only a silk cord should be used, and it should be examined frequently
to see that it has not got chafed. In swinging the thermometer, an open
place must be selected where it is not likely to come in contact with a
branch or any other object.
_Hygrometers._—As the humidity or degree of moisture in the atmosphere
is a very important climatic factor it is necessary to measure it as
carefully and as frequently as the temperature of the air. There are many
instruments, called psychrometers or hygrometers, for doing this; but few
of them are simple enough for the use of a traveller. The proportion of
water-vapour in air is a little difficult to understand at first, because
it is not a constant quantity as in the case of the other constituents of
air, but varies according to the amount of water-surface exposed to the
air and according to the temperature. The maximum amount of water-vapour
which can be present in air varies with the temperature, being greater
as the temperature is higher and less as the temperature is lower. Thus,
if air at 50° F. contains the maximum amount of water-vapour which it
can contain at that temperature, it is said to be _saturated_, for it
will take up no more and evaporation stops; and if the temperature were
to fall ever so little there would be more water-vapour present in the
air than it could hold and some would separate out and condense into dew
or rain, hence the temperature of saturation is called the dew-point.
But if air saturated at 50° is warmed up say to 60° it can then contain
more water-vapour than it has, and the temperature would require to
fall 10° before dew or rain could form. When the air is not saturated
water exposed to it evaporates rapidly until the maximum quantity of
water-vapour is again present, a larger quantity corresponding to the
higher temperature. At any given temperature the essential thing to know
about the humidity of the air is the additional amount of water-vapour it
could take up before becoming saturated, or in other words the humidity
relative to the maximum humidity possible at the existing temperature.
The relative humidity is expressed in percentages of the maximum humidity
possible (saturation) at the actual temperature of observation. It may
be measured by two methods, (1) finding the dew-point or temperature at
which the amount of vapour present saturates the air; (2) by finding the
rate at which the air allows evaporation to proceed; the farther the air
is from saturation the more rapid is the rate.
The dew-point may be found directly by means of an instrument by
which the air is cooled down until it begins to deposit moisture on a
polished surface, but such an instrument is inconvenient to handle when
travelling. It may also be found indirectly by calculation from the
relative humidity.
The relative humidity is most easily calculated from the rate of
evaporation. It is one of the laws of evaporation that heat is required
to change liquid into vapour, and when evaporation is going on heat is
being abstracted from surrounding bodies, and they are growing colder.
By allowing evaporation to take place from the bulb of a thermometer the
rate of evaporation may be measured by the fall of temperature produced,
and tables have been constructed to convert the differences between the
wet and dry bulb readings into relative humidities.
The wet-bulb thermometer consists of an ordinary thermometer, the bulb of
which is covered with clean muslin and kept moist by means of a piece of
cotton lamp-wick dipping into a small vessel of pure water. Care must be
taken to have the water quite pure and free from salt, otherwise the true
reduction of temperature will not be observed. Hence special precautions
are necessary when observing at sea or in an arid country where the
ground is covered with incrustations of salt.
In any form of wet bulb thermometer when the air is much below the
freezing point, it will usually be found most satisfactory to remove the
muslin covering and allow the bulb to become covered with a coating of
ice, by dipping it into water and allowing the water to freeze upon it.
Evaporation takes place from solid ice sufficiently rapidly to give the
true wet-bulb readings at least with a sling thermometer.
When the air is saturated, _i.e._, relative humidity = 100 per
cent., there is no difference in the reading of the wet and dry bulb
thermometers, and the greater the difference between the readings at a
given air temperature the smaller is the relative humidity of the air.
The wet-bulb thermometer has to be exposed to the air with the same
precautions as are taken in the case of the dry bulb. The two may
be hung side by side—but at least six inches apart—in the screen or
cage described on p. 15; or the wet bulb may be employed as a sling
thermometer. One way to do this is to tie a muslin cap on the bulb of the
sling thermometer with a piece of wet lamp-wick coiled round the upper
part of the bulb, and then whirl it until the reading becomes constant,
taking care to moisten the bulb again if it should become dry. Another
way is simply to twist a piece of filter-paper or blotting-paper round
the bulb, and dip it in water before swinging.
_Aspiration Psychrometer._—Perhaps the most convenient form of wet and
dry bulb thermometer for use by a traveller is that known as Assmann’s
Aspiration Psychrometer. It requires no protecting screen, is not subject
to the risk attending the use of the sling thermometer, and gives an
extremely close approximation to the true temperature and humidity.
The principle of the instrument is very simple. The wet and dry bulb
thermometers are enclosed separately each in an open tube (see Fig. 4)
through which a current of air is drawn by means of a fan, actuated by
clockwork in the upper part of the case. In making an observation, all
that is required is to see that the water vessel for the wet bulb is
filled and the bulb properly moist, and that the dry bulb is free from
any condensed moisture. The instrument is then hung to a branch or other
support placed in the open air (or even held in the hand), preferably in
the shade, although this is not essential, and the clockwork wound up.
Air will then be drawn over the bulbs for five minutes or more, and if
the temperature of each thermometer has not become steady by the time the
clockwork has run down, it must be wound up again.
[Illustration: FIG. 4.—_Section of Assmann’s Aspiration Psychrometer._]
The thermometers in Assmann’s Psychrometer are graduated according to the
Centigrade scale, and each degree is subdivided into fifths on a slip of
porcelain enclosed in the outer tube of the thermometer (see p. 13).
_Minimum Thermometer._—The minimum temperature of the night can usually
be ascertained by a traveller exposing a minimum thermometer when the
camp is set up and reading it in the morning before starting on his way.
There are several forms of minimum thermometer, but the only one likely
to be used is that known as Rutherford’s. It is very delicate and liable
to go out of order. The instrument should be of full size, as used in
meteorological stations at home; it must be packed so as to be as free as
possible from shock or vibration, and ought to be carried in a horizontal
position. The bulb is filled with alcohol or some similar clear fluid,
and within the column of spirit in the stem there is included a little
piece of dark glass shaped like a double-headed pin. This is the index
which continues pointing to the lowest temperature until the instrument
is disturbed or re-set. The thermometer has to be hung in a horizontal
position. When the temperature rises, the column of spirit moves along
the tube, flowing past the index without disturbing it. When the
temperature falls, the spirit returns towards the bulb, flowing past the
index _until the end of the column touches the end of the index_. The
phenomenon known as surface-tension gives to the free surface of any
liquid the properties of a tough film, and the smaller the area of a
free surface is, the greater is this effect of surface-tension. Hence it
is that the inner surface of the column of alcohol is not penetrated by
the glass index, but draws the index with it backwards towards the bulb.
As soon as the temperature begins to rise, the alcohol once more flows
past the index towards the farther end of the tube. The end of the index
farthest from the bulb remains opposite the mark on the stem indicating
the lowest temperature which had occurred since it was last set, and this
reading must be taken without touching the thermometer.
To set the index it is only necessary to tilt the bulb end of the tube
upwards, when the index will slide down by its own weight until it comes
in contact with the inner surface of the end of the column of alcohol.
_Care of a Minimum Thermometer._—The chief dangers to which a minimum
thermometer are liable are three—(1) the index being shaken into the
bulb, (2) the index being shaken partly or wholly out of the column of
spirit, and sticking in the tube, and (3) the column of spirit becoming
separated or a portion of the spirit evaporating into the upper end of
the tube.
The thermometer should be so constructed as to make it impossible for the
index to get into the bulb, or with an index so long as not wholly to
leave the tube, and this should be seen to before purchasing. When any of
the other derangements occurs the natural instinct of an observer is to
immerse the thermometer in warm water until the spirit entirely fills the
tube, and then allow it to cool. The only drawback to this simple method
is the almost inevitable bursting of the bulb and destruction of the
thermometer. This method should never be attempted; but if the warning
were not given, the idea would be sure to occur to the observer some time
or other, and he would proceed to destroy his thermometer with all the
fervour of a discoverer. The only satisfactory way to rectify a deranged
minimum thermometer is as follows:
If the column is separated, but the index remains in the spirit, grasp
the instrument firmly by the upper end and swing it downwards with a
jerk (as in the case of the mercurial thermometer mentioned on p. 23).
If the index has been shaken out of the spirit and remains sticking in
the upper part of the tube, or if a little spirit has volatilised into
the top of the tube and cannot be shaken down by the first method, a
quantity of spirit should be passed into the upper end of the tube by
grasping the thermometer by the bulb end of the frame and swinging in the
same way. When the index is immersed or the drop of volatilised spirit
joined on to the column, the first process of swinging by grasping the
upper end of the tube will bring the instrument into working order.
After any operation of this kind the thermometer should be kept in a
vertical position bulb downwards, to allow the spirit adhering to the
sides of the tube to drain back completely. Then the thermometer should
be brought into the horizontal position and set by allowing the index to
slide down to the end of the column of spirit. The end of the column of
spirit farther from the bulb should always show the same temperature as
the dry-bulb thermometer. If it should be observed to read a degree or
two lower, it will be found that some of the spirit has volatilized and
condensed at the end of the tube.
The minimum thermometer should be exposed to the air four or six feet
from the ground under a screen or roof, like that described on p. 15, so
that it is not exposed to the open sky, and the ground under the shelter
should be covered with grass or leaves, not on any account left bare. The
loss of heat by radiation of the ground to the open sky will produce a
night temperature much lower than that of the air a few feet above the
ground, and a radiation thermometer is often employed, laid on the grass
and exposed to the sky to measure this effect. Travellers, however, can
rarely be expected to make observations of such a kind, as the instrument
is one of extreme delicacy.
_Maximum Thermometers._—Maximum registering thermometers are
filled with mercury, and are less liable to get out of order than
spirit-thermometers. The simplest and best form for use by travellers is
Negretti and Zambra’s. Its principle is very simple. When the temperature
rises and the mercury in the bulb expands, it forces its way along the
stem in the usual manner; but there is a little constriction in the
tube just outside the bulb which breaks the column as the temperature
begins to fall, and so prevents the mercury in the bulb from drawing
back the thread of mercury from the tube. The thermometer is hung
horizontally, and the end of the mercury farthest from the bulb always
shows the highest temperature since it was last set. Before reading the
thermometer, it is well to take the precaution of seeing that the inner
end of the thread of mercury is in contact with the constriction in the
tube, and if, by the shaking of the instrument or otherwise, the mercury
has slipped away from this position, it should be brought back to it by
tilting the thermometer bulb downwards very gently, then returning it to
the horizontal position and reading.
To set this thermometer, it is only necessary to hold it vertically bulb
downwards and shake it slightly, if necessary striking the lower end of
the frame carrying the instrument, gently against the palm of the hand.
This causes the mercury to pass the constriction and re-enter the bulb.
When set, the end of the column farther from the bulb should indicate the
same temperature as the ordinary dry-bulb thermometer.
Another form of maximum thermometer is known as Phillips’. It is an
ordinary mercurial thermometer, but a short length of the upper part of
the column in the tube is separated from the rest by a little bubble of
air. It is used in the horizontal position, and as the temperature rises
the whole column moves forward, while, when the temperature falls, only
that portion behind the air-bubble retires towards the bulb. The tip of
the column thus remains to mark the maximum temperature to which its
farther end points. The instrument is set by gently tilting the bulb end
downwards, when the detached portion of the column at once runs back
until stopped by the air-bubble. This is the most convenient instrument
to use at a fixed station; but in travelling it is apt to get out of
order as shaking may have the effect of allowing the air-bubble to escape
into the upper part of the tube, or into the bulb, and the instrument
cannot easily be brought into working order again.
_Rain-Gauge._—While measurements of rainfall can possess no
climatological value unless they are carried on continuously at a
fixed station, some very interesting observations may be made by the
traveller both during the night when in camp, and during heavy showers
when compelled to stop on the march. The rain-gauge is in itself the
most simple of all scientific instruments, for it consists essentially
of a copper funnel to collect the rain as it falls, and a bottle to
contain what has been collected. A graduated measuring glass is the only
accessory required. Rain is measured by the depth to which the water
would lie on level ground if none soaked in, evaporated or flowed away.
On an emergency, a rain-gauge can be improvised out of a biscuit tin,
or any vessel with vertical sides and an unobstructed mouth. Such a
vessel standing level would collect the rain, the depth of which might be
measured by an ordinary inch-rule. It is rare, however, to find rain so
heavy as to give any appreciable depth when collected in a vessel freely
open to evaporation, and in order to estimate the amount of rainfall
to small fractions of an inch, the device is employed of measuring the
water collected in the receiver of the gauge in a glass jar of much
smaller diameter than the mouth of the collecting funnel. Thus, if the
funnel exposes a surface of fifty square inches, and the measuring glass
has a cross-section of one square inch, the fall of 1/50 of an inch of
rain on the funnel will give a quantity of water sufficient to fill the
measuring glass to the depth of an inch. In this way the actual rainfall
may be read to the thousandth part of an inch without trouble. The
smallest diameter for a serviceable rain-gauge is five inches, and this
size is well adapted for the traveller. A three-inch rain-gauge might
be employed, but the results obtained with it are not so satisfactory.
The rain-gauge should be placed in an open situation, so that it is not
sheltered by any surrounding trees or buildings, and it ought to be
firmly fixed by placing it between three wooden pegs driven securely
into the ground. The mouth of the gauge should be level, and when the
instrument is fixed, the rim of the funnel ought to be one foot above
the ground. A spare measuring glass should be carried, but as there is
always a considerable risk of breaking such fragile objects, it is well
to carry also one or two small brass measures of the capacity of half
an inch, two-tenths of an inch, and one-tenth of an inch of measured
rainfall. In this way, although no satisfactory record could be kept
of light rainfall, a very fair estimate may be made of any torrential
showers, the half-inch measure being used first, and then the smaller
measures, finally estimating by eye the fraction of the tenth of an inch
that remains over. It must, however, be distinctly borne in mind that an
estimate formed in this way is not an accurate measurement, and the fact
of using the rough method must be stated in the note-book.
When snow falls along with rain, the melted snow is measured as
equivalent to rainfall, and if the funnel of the rain-gauge should
contain some unmelted snow at the time of observation, it should be
warmed until the snow melts before a measurement is taken. When snow
falls in a strong wind the drift that occurs makes it almost impossible
to measure the amount accurately, but an effort should be made to
estimate the average depth of the snow over a considerable area.
If the receiving bottle of the rain-gauge should be broken by frost or
accident, any other bottle may be used, or in default of a bottle, the
copper case itself will act as a receiver, although the risk of loss by
evaporation, and by the wetting of a large surface in pouring out the
water, is considerably increased.
At a fixed station the rain-gauge should be read every morning. The
traveller who only exposes his rain-gauge during a halt should be careful
to state the hours when it was exposed and when it was read.
_Barometers._—The barometer is the most delicate, and at the same time
the most important, instrument which a meteorologist has to employ. It
requires particular care in transport, and must be very carefully mounted
and read, while several accessory observations have to be made at each
reading in order to ascertain the corrections required for the subsequent
calculation of the results. The function of the barometer is to measure
the pressure of the air at the time of observation, and this purpose may
be carried out by the use of two different principles. The oldest and
best method is to measure the height at which a column of heavy fluid is
maintained in a tube entirely free from air. The weight of this column is
equal to the weight of a column of the atmosphere of the same sectional
area. Mercury being the densest fluid is the only one usually employed,
because the column balancing a column of the atmosphere of equal
sectional area is the shortest that can be obtained, and, consequently,
a mercurial barometer is the most portable that can be constructed on
this principle. The mercurial barometer has come to be recognised as the
standard in all parts of the world.
The average height of the column of mercury in a barometer is about
thirty inches, and, consequently, the whole instrument cannot well be
made less than three feet long, so that when account is taken of the
glass tube, and the amount of mercury it contains, it is long, fragile
and heavy. To avoid the disadvantages inherent in such an instrument,
the method of measuring the pressure of the air by the compression of a
spring holding apart the sides of an air-free flexible metallic box was
devised, and the aneroid barometer invented. The aneroid is graduated
on the dial in “inches,” _i.e._, divisions each of which corresponds to
a change of atmospheric pressure, equal to that measured by one inch of
mercury in a standard barometer. Although a carefully constructed aneroid
is a very useful instrument indeed, it is not to be trusted like a
mercurial barometer kept in a proper place. But a good aneroid is likely
to be much more serviceable to the ordinary traveller on the march than
a standard mercurial barometer, every packing and unpacking of which
exposes it to the risk of breakage, or to the equally fatal risk of air
obtaining access to the vacuous space at the top of the tube. The scale
of a barometer may be divided into millimetres, or, as now recommended
by the British Meteorological Office, into millibars or thousandths of
a hypothetical “atmosphere.” We shall describe the Fortin barometer,
which is best adapted for use at a fixed station, and one devised by
Prof. Collie and Capt. Deasy, which is portable enough for the use of
travellers.
_The Fortin Barometer._—The barometer must be kept in a room with as
equable a temperature as possible; the instrument must be absolutely
vertical—hence it should be hung freely and not touched while it is being
read; it must be in a good light, and yet be sheltered from the direct
rays of the sun. The measurement of the height of any mercurial barometer
is that of the difference of level between the surface of the mercury in
the tube and the surface of the mercury in the cistern. When the mercury
rises in the tube it falls in the cistern, and _vice versâ_, although
when the cistern is much wider than the tube the changes of level there
are much less than those in the tube. In most barometers an arbitrary
correction is made to allow for this change, the “inches” engraved on
the scale not being true inches. In the Fortin barometer, however, the
lower end of the measuring rod is brought in contact with the mercury in
the cistern before every reading, and then the scale of inches engraved
on the upper part of the measuring rod gives the true height of the
column of mercury. In calculating the barometric pressure for purposes
of comparison, five corrections have to be applied: (1) for temperature,
which requires the temperature of the barometer at the time of reading to
be observed, (2) for altitude, which necessitates knowing the elevation
of the place of observation above sea-level, (3) for the force of gravity
at sea-level, which requires the latitude to be known, (4) for the
capillary attraction between the mercury and the glass tube, which is a
constant for each barometer, (5) for the slight imperfection in engraving
the scale (index error), which is also a constant for each instrument.
[Illustration: FIG. 5.—_Two Readings of the Barometer Vernier._]
It is enough for the observer at a fixed station, and to such alone can
the use of a Fortin barometer be recommended, to read the temperature on
the thermometer attached to the barometer and to read the height of the
mercury in the barometer tube. These two figures he is to enter in his
note-book, and unless he is himself discussing the results, he should
apply no correction whatever to them. The rules for observing, then, are:—
1. Read the attached thermometer and note the reading.
2. Bring the surface of the mercury in the cistern into contact with the
ivory point which forms the extremity of the measuring rod by turning the
screw at the bottom of the cistern. The ivory point and its reflected
image in the mercury should appear just to touch each other and form a
double cone.
3. Adjust the vernier scale so that its two lower edges shall form a
tangent to the _convex_ surface of the mercury. The front and back edges
of the vernier, the _top_ of the mercury, and the eye of the observer are
then in the same straight line.
4. Take the reading, and _enter the observation as read_ without either
correcting it to freezing point or reducing it to the sea-level.
The scale fixed to the barometer is divided into inches, tenths, and
half-tenths, so that each division on this scale is equal to 0.050 inch.
The small movable scale or vernier attached to the instrument enables
the observer to take more accurate readings; it is moved by a rack and
pinion. Twenty-four spaces on the fixed scale correspond to twenty-five
spaces on the vernier; hence each space on the fixed scale is larger than
a space on the vernier by the twenty-fifth part of 0.050 inch, which is
0.002. Every long line on the vernier (marked 1, 2, 3, 4, and 5) thus
corresponds to 0.010 inch. If the lower edge of the vernier coincides
with a line on the fixed scale, and the upper edge with the twenty-fourth
division of the latter higher up, the reading is at once supplied by
the fixed scale as in A (Fig. 5), where it is 29.500 inches. If this
coincidence does not take place, then read off the division on the fixed
scale, above which the lower edge of the vernier stands. In B (Fig. 5)
this is 29.750 inches. Next look along the vernier until one of its lines
is found to coincide with a line on the fixed scale. In B this will be
found to be the case with the second line above the figure “2.” The
reading of the barometer is therefore:—
On fixed scale 29.750
On vernier (12 × .002) .024
------
Correct reading 29.774
Should two lines on the vernier be in equally near agreement with two on
the fixed scale, then the intermediate value should be adopted.
5. Lower the mercury in the cistern by turning the screw at the bottom
until the surface is well below the ivory point; this is done to prevent
the collection of impurities on the surface about the point.
The transport of barometers requires very great care in order to prevent
the introduction of air into the tube or the fracture of the tube by
the impact of the mercury against the top. To reduce the risk of these
accidents, the barometer must be carried with the tube quite _full of
mercury_, and in an inverted position, at least with the cistern end kept
higher than the top of the tube. The flexible cistern of the Fortin type
of barometer allows of it being screwed up tight so as to fill the tube
and close the lower end of it. In case of breakage, the operation of
fitting a new tube is not very difficult, but unless the tube has been
carried out ready filled with mercury, this cannot well be attempted. In
order to drive out the film of air adhering to the glass on the inside,
it is necessary, after filling the tube, to raise its temperature to
the boiling-point of mercury. No one should attempt either to fill or
to change a barometer tube unless he has had practice in doing so under
expert supervision beforehand.
_The Collie Portable Mercurial Barometer._—This instrument is not likely
to be broken in travelling. It is quickly set up, and from such tests
as have been applied, it appears to give excellent results. The cistern
and vacuum tube at the top are of equal diameter, and are connected by a
flexible tube, and the difference in level of the mercury may be measured
directly by means of a graduated rod, or as in Deasy’s mounting by means
of a vernier. There is no attached thermometer, but if the instrument be
used in the open air, and is exposed for ten minutes or a quarter of an
hour before using, it will be sufficient to note the temperature of the
air in the usual way.
The upper end (Fig. 7) is about 2.5 inches long, and contains an
air-trap, into which all the air that may accidentally enter the
barometer, either by the tap leaking, through the rubber tubing, or
through either of the joints, must find its way. The lower or reservoir
end (Fig. 8) is about 4.5 inches long, and has an air-tight glass tap
about an inch below the broad part. These ends are forced into the rubber
tubing, and, as an additional precaution against leakage, copper wire is
bound round the joints. The scale is cut on an aluminium bar, along which
two carriages, to which the barometer is attached, move up and down, and
they can be clamped to the bar at any place (Fig. 6). By means of the
verniers attached to the carriages, which are divided to 0.002 of an
inch, it is easy to estimate the height of the mercury to 0.001.
To use the barometer, the carriages are put on the scale bar; the lower
one is clamped at the bottom of the bar, and the upper one some inches
higher up; the barometer is attached to the carriages by clamps which fit
over the joints; the rubber cap is removed from the reservoir end, the
tap opened, the verniers put in the middle of their runs, and the upper
carriage moved up the bar until there is a vacuum. By means of the screws
on the right of the carriages the verniers are moved up or down until the
top of the mercury at each end is in line with the edges of the rings
attached to the verniers, which fit round the glass ends. Both verniers
are then read, and the difference gives the height of the barometer. The
rubber cap on the reservoir end is merely to prevent the small quantity
of mercury, which should be left above the tap when it is closed, from
being shaken out when travelling.
[Illustration: FIG. 6. THE COLLIE BAROMETER, WITH THE DEASY MOUNTING, IN
ITS NORMAL WORKING POSITION.
FIG. 7. THE UPPER CARRIAGE AND VERNIER ON A LARGER SCALE, WITH BAROMETER
ATTACHED.
FIG. 8. THE LOWER CARRIAGE AND VERNIER, WITH RESERVOIR END OF BAROMETER
ATTACHED. (Same scale as Fig. 7.)]
To pack up the barometer, lower the upper carriage _very slowly_ until
the mercury has touched the top of the glass; then detach the barometer
from this carriage, and either let the upper end hang vertically below
the reservoir, or detach the reservoir end from its carriage and raise
it till the barometer hangs vertically. By this means the barometer is
completely filled with mercury, and then the tap must be closed. The tube
is then to be coiled away in its padded box. When too much air is found
in the trap, it must be extracted by means of the air-pump.
_The Aneroid Barometer._—The aneroid barometer is so convenient on
account of its portability that, although much less trustworthy than a
mercurial barometer, it is much more likely to be used by a traveller.
Care should be taken in using it to see that the pointer has come to a
position of equilibrium, and it should be tapped gently before reading.
The eye must be brought directly over the end of the pointer, and the
reading made to one-hundredth of an inch, the barometer being held in
a horizontal position. Every opportunity of comparing the aneroid with
a standard mercurial barometer should be taken, and a note made of the
readings of both. The mercurial barometer will require to be corrected
for temperature before its indications can be used for correcting the
aneroid, as all good aneroids are compensated for changes of temperature.
The readings of an aneroid give a very fair idea of the changes of
atmospheric pressure, and are very much better than none at all, although
they cannot in any case be accepted as of the highest order of accuracy.
The Watkin mountain aneroid, which is so constructed as to be thrown into
gear at the moment when it is read, appears to be free from the worst
errors of the ordinary aneroid.
For climatological purposes, it is impossible to make barometric
observations of value while travelling unless the altitude of each
camping-place is accurately known. This is practically never the case
except when travelling along the sea-shore or the margin of a great lake
the elevation of which has been determined. But, meteorology apart,
barometric readings in any little known country are of value, because by
comparing them with simultaneous readings taken at a neighbouring fixed
station, new data as to the altitude of the country may be obtained.
While in camp, it would be an extremely useful thing to make barometer
readings, even with an aneroid, every two hours, in order to get some
information as to the normal daily range of atmospheric pressure.
_The Boiling-point Thermometer._—The temperature at which water
boils depends on the pressure of the atmosphere, so that an accurate
observation of the boiling-point of water enables the pressure of the
atmosphere at the moment of observation to be determined with the utmost
accuracy. This method of determining atmospheric pressure having been
used hitherto almost solely for the purpose of measuring altitudes, the
boiling-point thermometer is usually known as the Hypsometer, but its
records are quite as valuable for use at fixed stations as in mountain
climbing. Mr. J. Y. Buchanan recommends the use of a boiling-point
thermometer with a very open scale graduated to fiftieths of a degree
Centigrade and entirely enclosed in a wide glass tube through which
steam from water boiling in a copper vessel is passing. On a thermometer
of this kind change of pressure can be measured by the change of
boiling-point more accurately than with the aid of a mercurial barometer.
See Table, Vol. I., p. 293.
2. OBSERVATIONS FOR FORECASTING THE WEATHER.—The familiar name of
“weather-glass” is appropriately applied to the barometer, for in most
parts of the world it is the surest indicator of any approaching storm.
The scientific prediction of the weather by means of the barometer
involves the comparison of the simultaneous readings of barometers over
as wide an area as possible, and can only be carried out where there is
a complete telegraph system and a public department charged with the
work. The storms of wind and rain which break the more usual steady
weather are usually associated with the formation of centres of low
atmospheric pressure towards which wind blows in from every side. These
atmospheric depressions move, as a rule, in fairly regular tracks, the
rate of movement of the centre of the depression having no relation to
the rate at which the wind blows or to the direction of the wind. The
term _cyclone_ is usually applied to such a moving depression, because of
the rotating winds round the centre; but the size of a cyclone may vary
from a vast atmospheric eddy extending across the whole breadth of the
Atlantic to one only a few miles in diameter. The strength of the wind in
a cyclone depends on the barometric gradient; in other words, the greater
the difference in atmospheric pressure between two neighbouring points
the stronger is the wind that blows between them. Or, when a cyclone is
passing over an observer, the more rapidly the barometer falls _or rises_
the stronger may the wind be expected to blow.
In direct contrast to the cyclone or depression is the system of high
pressure rising to a centre from which the wind blows out on every
side. This is called an anticyclone, and is a condition which, once
established, may last for many days, or even weeks, without change. It is
the typical condition for dry calm weather in all parts of the world.
[Illustration: FIG. 9.—CYCLONE PATHS AND CIRCULATION OF WINDS IN CYCLONES
IN THE NORTHERN AND SOUTHERN HEMISPHERES.]
The direction of movement of the centres of cyclones in the northern
hemisphere is usually westward and northward near the equator, the
path of the centre bending to the right as it proceeds, and becoming
ultimately eastward and southward. In the southern hemisphere the
direction of the centre near the equator is westward and southward,
turning towards the left as it proceeds. The rotation of the wind about
the centre of a cyclone in the northern hemisphere is inwards towards
the centre in the direction opposite to the hands of a watch, and in the
southern hemisphere it is in the direction in which the hands of a watch
move (Fig. 9). In the centre of a cyclone there is a calm, a well-known
danger to sailing ships caught in such a storm at sea, because there is
no wind to move the vessel, but a tremendous sea driven in from the gale
which rages all round from every point of the compass. The law of storms
has been very fully studied, and rules have been drawn up to enable
sailors to ascertain the direction in which the centre of an approaching
cyclone lies and the direction in which it is moving. In a work intended
mainly for travellers on land it is not necessary to give these rules;
all that is required is to tell how the approach of dangerous storms may
be ascertained some time in advance. The fact that the barometer is high
or low is in itself of no value for prediction. The important thing to
know is the distribution of atmospheric pressure at a given moment over a
considerable area. To the isolated observer this is impossible, and he
can only judge of the state of the atmosphere by observing the _rate_ at
which the barometer is falling or rising. Thus, if for several days the
barometer has been steadily and slowly rising, he will probably be right
in believing that an anticyclonic condition is establishing itself, and
that the weather may be expected to continue fine for many days to come,
even after a gradual fall of the barometer begins. A sudden fall of the
barometer, on the other hand, is always a sign of wind, and usually of
wet weather as well. This is a particularly valuable sign of approaching
storm in those parts of the world where, as in the tropics, the normal
weather is very uniform and steady. In such places a very sudden fall,
say one-tenth of an inch in an hour, is a sure precursor of a violent
storm. As the barometer continues to fall, the wind will probably
continue to increase in force, and when the barometer reaches its lowest
point it will either fall calm (if the centre of the cyclone is passing
over the observer) or suddenly change in direction. The rapid rise of the
barometer after a great depression is also always accompanied by strong
wind, though not so frequently by rain.
It must be clearly understood that these remarks refer only to
observations at a fixed station. If a fall of barometer is observed
in travelling, it may be due either to a change in the state of the
atmosphere or to a change in the traveller’s height above sea-level.
This is the reason why it is absolutely essential, in making barometric
estimates of height (or boiling-point determinations), to have
simultaneous observations going on at a base-station, or preferably at a
series of intermediate stations.
The ordinary prognostics of the approach of rain or bad weather differ
in different localities, and require a considerable amount of local
knowledge before they can be utilised. The peculiar absorption band in
the solar spectrum due to the water vapour of the Earth’s atmosphere,
and called the rain-band, is a valuable guide to an experienced observer
with a spectroscope in predicting rain. The only instrument, however,
likely to be useful to the ordinary traveller is the wet and dry bulb
thermometer. When the two thermometers have the same reading, indicating
saturation of water vapour, or when they approach at temperatures above
60° F. within two degrees or so, rain may be expected, or possibly mist.
The appearance of low clouds clinging to the hillsides is an indication
that the temperature at the place where they are is below the dew-point.
The appearance of the upper clouds, taken in conjunction with the
readings of the barometer, is a valuable indication of forthcoming
weather changes. The increase of cirrus clouds in a clear sky with a
falling barometer, or the appearance of a solar or lunar halo, may be
taken as a sure sign of an approaching cyclone, the intensity of which
may be foreseen by the _rate_ at which the barometer is falling.
While the weather of places on the west coasts of temperate continents
exposed to the prevailing sea-wind is usually made up of a succession
of cyclones of different degrees of intensity, and of the anticyclonic
intervals between them, over the greater part of the Earth’s surface the
climate is much more uniform, and the seasonal changes are the principal
cause of changes of weather. To understand these general conditions it is
necessary to consider the elements of climatology.
3. OUTLINES OF CLIMATOLOGY.—The air is in constant movement on account
of the unequal way in which the heat of the sun falls on different
parts of the Earth’s surface, and at different seasons of the year. All
the conditions of the atmosphere show a certain diurnal periodicity
which is most marked in the regions of steady climate between and
near the tropics. Thus, as a rule, the minimum temperature of the air
occurs just before sunrise, the maximum temperature from two to three
hours after noon. The amount of difference between the maximum and
minimum temperature of the day (daily range) is least near the sea or
in wet regions (a maritime climate) and greatest in the interior of
the continents, especially where the rainfall is slight (a continental
climate). Over the sea itself the daily range of air temperature averages
only 3 Fahrenheit degrees; but in the heart of a continent, especially in
a desert, it may exceed 60 Fahrenheit degrees.
Diurnal changes of pressure are proportionally much smaller in amount
than changes of temperature, and are to be observed as a regular
phenomenon only in the tropics, or elsewhere during very settled weather.
There are usually two maxima daily, about 10 A.M. and 10 P.M., and two
minima occurring about 4 A.M. and 4 P.M. It is only in rare cases that
the total barometric range exceeds 0.10 inch, very frequently it is not
greater than 0.04 inch. Still it is convenient to remember in the tropics
that a fall of the barometer not greater than 0.10 inch between 10 A.M.
and 4 P.M. is to be expected, and does not indicate either the approach
of a storm (if the observer is at rest) or the ascent of 100 feet (if he
is on the march).
Associated with the diurnal changes of temperature in settled weather are
changes of wind due to local configuration of the ground. The wind, for
example, usually blows up a mountain side, or up a steep valley, during
the day, and down a mountain, or down a steep valley, during the night.
So, too, the regular land and sea breezes found on the borders of the sea
or of great lakes blow from water to land in the day time and from land
to water at night. Here the determining cause is the fact that land is
warmed and cooled by radiation, and in turn heats or chills the air much
more than water does. In the settled climates of high tropical plateaus
a regular diurnal change of wind direction has been observed, the wind
blowing successively from all points of the compass.
A similar diurnal periodicity occurs in the amount of cloud, in the
moisture of the air, the fall of rain, the occurrence of thunderstorms,
etc. It is also to be noticed in the flow of rivers in mountainous
regions where the streams take their rise from glaciers or snow, the
rapid melting of which by the heat of the sun causes the volume of water
to increase greatly in the afternoon, while the cessation or reduction of
the rate of melting at night diminishes the volume of the river in the
morning and forenoon.
Periodic changes of greater amount but similar in kind are produced by
the alternation of the seasons, the difference between the mean values
of the months in which the phenomena are at a maximum and minimum
respectively being termed the annual range. With regard to temperature,
very moderate changes occur in the tropical zones where the altitude
of the noon-day sun is always great, and the length of day and night
varies little with the season (for the most part less than 5 Fahrenheit
degrees); but in the temperate and frigid zones there are strongly marked
annual changes. As in the case of daily range, proximity to the sea is
a controlling factor in the annual range of temperature. To take a very
characteristic instance, the annual range between the mean temperature of
July and January is about 23 Fahrenheit degrees in the Lofoten islands
on the margin of the Atlantic, while it is 120 Fahrenheit degrees at
Verkhoyansk in the same latitude, but in the centre of the Asiatic
continent.
The extreme months for air temperature are January and July in almost
every part of the world, the maximum occurring north of the equatorial
belt in July and south of it in January.
The annual changes in barometric pressure and wind are equally marked.
The belt of low pressure which lies nearly under the vertical sun moves
northward over the surface of the globe in the northern summer, coming
to its most northerly position in July: returning southward after the
sun, it reaches its most southerly position in January. This belt of
low pressure is also a belt of calms, known by sailors as the Doldrums,
and it is a belt of frequent rains, so that as it approaches and passes
over a place there is a rainy season, followed by a dry season when it
retires. Near the mean position of the belt of low pressure, where it
passes over a place twice in the year, there are two rainy seasons.
The low pressure belt is bordered to north and south by belts of high
atmospheric pressure, from which the trade-winds blow towards the
equator, and the westerly anti-trades blow towards the poles. These are
also subject to the annual change; but the different action of land and
sea on the distribution of pressure exercises a greater influence than
does the difference of latitude. As the greater heating and cooling of
the land each day causes the phenomena of daily land and sea breezes,
so the greater heating and cooling of the land between summer and
winter causes seasonal land and sea winds, blowing from land to sea in
winter, from sea to land in summer. Generally speaking, the pressure is
greater—in the same latitude—where the air is cooler, so that outside the
frigid zones cold areas are usually areas of high pressure, from which
wind blows out in every direction, while warm areas are areas of low
pressure towards which wind blows in on every side.
The distribution of rainfall on the land is dependent on the direction
of the rain-bringing wind and the configuration of the surface. Thus
when the rain-bringing wind meets a mountain range, it deposits a great
rainfall on the exposed slopes, but passes over as a dry wind which
yields little rain to the region beyond. In places where the wind changes
with the season, as in southern Asia, the distribution of rainfall is
entirely different during the continuance of the different monsoons.
All these questions of normal climate can be more easily illustrated on
maps than explained by words. But the reader must be cautioned against
taking the condensed and generalised representations of small-scale maps
as showing all that is known on the subject. Even the magnificent plates
in the ‘Atlas of Meteorology,’ which forms part of Bartholomew’s Physical
Atlas, cannot show everything that is known; and in many parts of the
world so little has yet been ascertained as to the climatic conditions
that generations of observers will be required to make it possible for
meteorologists to draw a uniform trustworthy map of the whole world
showing the distribution of any one element of climate.
_Isothermal maps._—The principle of an isothermal map is that of
representing the distribution of temperature by drawing lines through
all the places where the temperature is the same at a given time. It is
usual to take this time as an average month in an average year. Thus in
a map of isotherms for January (see p. 50), what is shown is not the
temperature of any particular day in any particular January, but that
of an average day in a long series of Januaries. Hence it is not likely
that the _exact_ distribution of temperature shown in the map will
ever be found on any January day; but it is to be expected that most
days in every January will have a distribution of temperature which is
very similar to that shown. The same is of course true of maps showing
pressure, or rainfall, or any other average condition.
Again, the isotherm is necessarily constructed from average temperatures
which have been corrected so as to be applicable to the same level. On
the equator, for instance, the summit of a lofty mountain is seen by
the snow on it to have a temperature not exceeding 32° F., while at
sea-level the temperature may be 90°. But observations have been made
showing the rate at which the temperature of the air diminishes as the
height increases, and although the rate varies in different places and
at different seasons, it may be taken roughly as one Fahrenheit degree
in 300 feet. Now if the mountain top with a temperature of say 30° F.
is known to be 18,000 feet above the sea, the addition of 1° for every
300 feet, or 60° altogether, would give the temperature of 90° as that
corresponding to sea-level. By applying such corrections, the isothermal
maps have been constructed to show the distribution of temperature at the
level of the sea. In order to compare the temperature he has observed
with that on the map the observer must calculate the average of his daily
observations for the month in question, and then make the correction for
the altitude of his station.
Similarly, in ascertaining from an isothermal map the mean temperature
of a particular place, care must be taken to subtract from the number
of degrees of the isotherm passing through the place one degree for
every 300 feet of elevation. Of course it will usually happen that no
isotherm as shown on the map runs through the point the mean temperature
of which it is desired to obtain. In that case the temperature at the
point will be found by considering its relative position between the two
nearest isotherms. Thus, if it lie half-way between the lines of 60°
and 70°—measured perpendicularly to the isotherms—the temperature of
65° may be assumed; if it lies one-tenth of the distance from 60° and
nine-tenths from 70°, it is safe to assume 61°; if three-tenths from 70°
and seven-tenths from 60°, then assume 67°. If the point lie in a loop
of a single isotherm, _e.g._, Cape St. Roque, the eastern point of South
America in the map for January, lying within the 80° isotherm, one can
only guess that the temperature is above 80° and it may be assumed to be
below 85°. The method of representation is unsatisfactory in such a case.
These facts being borne in mind, the study of isotherm maps will be found
to give an excellent general idea of the distribution of climate at
sea-level, and if the contour lines of 600 and 6000 feet are traced on
the maps the areas within which corrections of over -2° and -20° have to
be applied to the isothermal values to get the temperature at the place
will be easily recognised.
_Isobaric Maps._—Isobars are drawn from the data of the height of the
barometer corrected to sea-level values and to the temperature of 32°
F., exactly in the same way as isotherms are drawn from the data of
thermometer readings or contour lines from data of altitude measurements.
The practical value of the study of isobars is very great, because of the
importance of assuming a probable value of sea-level pressure in reducing
the barometric or boiling-point thermometer readings for determining
elevation, and also because of the intimate relation between the form and
proximity of isobars and the direction and force of the winds.
Barometric gradient is measured by the difference between the isobars per
unit of length. For instance, gradient is frequently expressed in the
number of hundredths of an inch difference between barometers fifteen
nautical miles apart. The greater the gradient of pressure is, the more
closely together must the isobars be drawn in order to represent it. For
example, in the isobaric map for January (p. 50) a very steep gradient is
shown on the east coast of Asia, north of Japan, and a remarkably gentle
gradient in the interior of Asia from the Black Sea eastward. The steeper
the gradient the stronger is the wind.
The arrows in the isobaric maps (which are represented flying with the
wind) show the average directions of the wind over the world for the
months in question. The relation they bear to the isobars becomes clear
on inspection, although, on account of the greater number of observations
available for some parts of the world than for others, all the arrows are
not drawn with the same amount of certainty, and the direction of a few
contradicts that of most. As a general rule, the following facts may be
taken as absolutely established: (1) Wherever there is a region of high
pressure the wind blows out from it in all directions. (2) Wherever there
is a region of low pressure the wind blows in towards it from every side.
(3) The wind never blows perpendicularly to the isobars or directly from
higher to lower pressure, but always in a curved or spiral path inclined
to the isobars. (4) In the northern hemisphere the wind blows out from
a high pressure area in the same direction as the hands of a watch
move, but in the southern hemisphere in the opposite direction. Also in
the northern hemisphere the wind blows into a low-pressure area in the
direction opposite to that of the hands of a watch and in the southern
hemisphere in the same direction as the hands of a watch move. (5)
Recognising that the wind blows nearly parallel to the direction of the
isobars, the following statement (known as Buys Ballot’s Law) expresses
its direction both for high-pressure and for low-pressure areas: If you
stand with the lower pressure on your left hand, and the higher pressure
on your right hand, in the northern hemisphere the wind will be blowing
on your back, but in the southern hemisphere in your face.
_Rainfall Maps._—Rainfall is represented on maps by lines of equal
precipitation termed Isohyets. These represent actual figures without
reduction for elevation or other local conditions, and a rainfall map
can consequently be studied as a direct record of observed facts. The
map (p. 50) of mean annual rainfall brings out clearly the equatorial
zone of heavy rains crossing the Amazon valley, the Congo valley, the
southeastern peninsulas of Asia and the Malay archipelago. North and
south of this belt are the nearly rainless regions of the tropical
deserts, extended northward and southward over the continents, and
merging nearer the poles into the fairly-watered temperate zones. The
rainfall maps for separate months show the intimate relation between
rainfall and the direction of the wind taken in conjunction with the
configuration of the land. Even on the coast, when the prevailing wind
is off shore, there may be scarcely any rain, as on the west coast of
tropical South America. In the very heart of a continent the rainfall
may be very heavy where the sea-wind blows across a great plain before
striking the mountains, as is illustrated by the eastern slope of the
Andes. Rainfall is, however, one of the most inconstant elements of
meteorology, and the actual rainfall of any year may differ very widely
from the average. The practical value of exact statistics of rainfall
is, however, greater than that of any other climatological condition; for
the water supply and the fertility of the land depend in every case on
the rain that falls either locally or on the heights of the water-sheds.
In order to pursue the subject further the chapters on the atmosphere in
the writer’s ‘Realm of Nature’ (London, Murray; New York, Scribner) and
Mr. L. C. W. Bonacina’s ‘Climatic Control’ (London, A. and C. Black) may
be useful. The most systematic treatment of climatology will be found in
Hann’s ‘Handbuch der Klimatologie,’ 3 vols. (Stuttgart), which contains
numerous references to special works; the essential part of this treatise
is translated by Professor R. de C. Ward, under the title of ‘Handbook of
Climatology,’ Part I. (Macmillan). The most important work of all is the
great ‘Atlas of Meteorology’ by Dr. A. Buchan and Dr. A. J. Herbertson,
forming Vol. III. of Bartholomew’s ‘Atlas of Physical Geography’ (London,
Constable), which gives an unrivalled series of climate and weather-maps
with explanatory letterpress.
The recent great advances in meteorology have rendered the old textbooks
obsolete, while the new data, especially those regarding the upper
regions of the atmosphere, have not yet been built into a coherent
system. Sir Napier Shaw’s ‘Forecasting Weather’ (London, Constable &
Co.) is a pioneer discussion, and the numerous publications of the
Meteorological Office, South Kensington, London, S.W.7, may be consulted
with much advantage.
Daily synoptical weather-maps are published by the Weather Service of
almost every civilised country. Those for the United Kingdom may be
obtained from the Meteorological Office of the Air Ministry. The only
weather-maps of large areas produced regularly are the Pilot Charts
of the North Atlantic and the North Pacific, published monthly by the
Hydrographic Office at Washington, and those of the North Atlantic and
Mediterranean published monthly by the Meteorological Office in London.
These show the tracks of cyclones, and give a great deal of information
as to the meteorology and currents of the oceans. They are intended
primarily for the use of sailors.
The following list gives the name of the official weather service of
all countries outside Europe and the town in which the head office
is situated. Application might be made to any of these offices for
information as to the stations where standard instruments are established
in the country in question.
EXTRA-EUROPEAN WEATHER SERVICES.
===================+====================================+================
Country. | Name. | Place.
===================+====================================+================
_America_:— | |
| |
Canada | Meteorological Service of the |
| Dominion of Canada | Toronto.
United States and | |
West Indies | Weather Bureau | Washington.
Mexico | Observatorio meteorologico |
| magnetico central | Mexico.
Salvador | Observatorio Astronomico y | San Salvador.
| Meteorologico |
Guatemala | Instituto fisico-geografico | San José.
Cuba | Real Colegio de Belen de la |
| compania de Jesus | Havana.
Brazil | Central Meteorological Department |
| of the Navy | Rio de Janeiro.
Uruguay | Observatorio meteorologico del |
| colegio pio de Villa Colon | Montevideo.
Argentina | Oficina meteorologica Argentina | Buenos Aires.
Chile | Oficina meteorologica | Santiago.
Peru | Harvard College Observatory | Arequipa.
| |
_Africa_:— | |
| |
Algeria | Service météorologique | Algiers.
British Central | |
Africa | | Zomba.
Egypt | Observatory | Helwan.
Madagascar | Observatoire | Tananarive.
Mauritius | Royal Alfred Observatory | Pamplemousses.
Union of South | Meteorological Branch, Irrigation |
Africa | Department | Johannesburg.
| |
_Asia_:— | |
| |
India | Meteorological Department | Calcutta.
Japan | Central Meteorological Observatory | Tokyo.
Ceylon | Surveyor-General’s Office | Colombo.
Hongkong | Observatory | Hongkong.
China | Imperial Maritime Customs | Peking.
” | Observatoire de Zi-ka-wei | Shanghai.
Philippine Islands | Weather Bureau | Manila.
Dutch East Indies | Magnetic and Meteorological | Batavia.
| Observatory |
| |
_Australasia and | |
Pacific_:— | |
| |
Australia | Commonwealth Meteorological Service| Melbourne.
New Zealand | Meteorological Office | Wellington.
Hawaii | Weather Bureau | Honolulu.
===================+====================================+================
In addition to the above, which are regular government services specially
organised for publishing and utilising the data from numerous observing
stations, there are many isolated stations in all parts of the world.
There are no colonies which do not possess some meteorological stations,
and at many mission stations meteorological observations are made. It
would always be well for a traveller to try to ascertain where in the
vicinity of his route meteorological stations have been established and
over what period of time their records extend.
In the Table (pp. 44-49), taken from Marriott’s “Hints to Meteorological
Observers,” is given the relative humidity for every 2° of temperature
from 20° to 80°, and for every two-tenths of a degree of difference
between the dry and wet-bulb readings from 0°·2 to 18°·0.
To use the Table: Look in the column on the left or right for the nearest
degree to the dry-bulb reading; then carry the eye horizontally along
until the column is reached corresponding to the difference between the
readings of the dry and wet-bulb thermometers, when the relative humidity
will be found. Intermediate readings can be interpolated in the usual way.
_Example:_ Dry-bulb 58°·5, wet-bulb 52°·7, the difference is 5°·8. Having
found 58° in the column on the left or right, run the eye along this line
until the column under 5°·8 is reached, when the relative humidity will
be found, viz., 67.
TABLE OF RELATIVE HUMIDITY.
======+=================================================================
Dry Bulb Reading.
| DIFFERENCE BETWEEN THE READINGS
| OF THE DRY AND WET BULB THERMOMETERS.
| ° ° ° ° ° ° ° ° ° ° ° ° °
| 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6
======+=================================================================
° |
20 | 93 86 80 74 68 63 59 55 51 47 44 41 37
22 | 94 88 82 76 70 65 60 56 53 50 47 44 41
24 | 94 88 83 78 73 69 65 61 57 53 50 47 44
26 | 95 90 85 80 76 72 68 64 61 57 54 51 48
28 | 96 92 88 84 80 76 72 69 66 63 60 57 54
30 | 96 93 90 87 84 81 78 75 72 70 67 64 62
32 | 97 94 91 89 87 84 82 80 78 76 73 71 69
34 | 97 95 93 91 89 87 85 83 81 80 78 76 74
36 | 98 96 94 93 91 89 87 86 84 82 80 79 77
38 | 98 96 94 93 91 89 87 86 85 83 81 80 78
40 | 98 97 95 94 92 90 88 87 86 84 82 81 79
42 | 98 97 95 94 92 90 88 87 86 84 83 81 80
44 | 98 97 95 94 92 90 88 87 86 84 83 82 81
46 | 99 97 95 94 92 91 89 88 87 85 84 82 81
48 | 99 97 95 94 92 91 89 88 87 85 84 82 81
50 | 99 97 96 94 93 92 90 89 88 86 85 83 82
52 | 99 97 96 94 93 92 90 89 88 86 85 83 82
54 | 99 97 96 94 93 92 90 89 88 86 85 83 82
56 | 99 97 96 94 93 92 90 89 88 87 86 84 83
58 | 99 97 96 94 93 92 90 89 88 87 86 85 84
60 | 99 97 96 94 93 92 91 90 89 88 86 85 84
62 | 99 98 96 95 94 93 91 90 89 88 86 85 84
64 | 99 98 96 95 94 93 91 90 89 88 86 85 84
66 | 99 98 96 95 94 93 91 90 89 88 87 86 85
68 | 99 98 96 95 94 93 92 91 90 89 87 86 85
70 | 99 98 96 95 94 93 92 91 90 89 87 86 85
72 | 99 98 96 95 94 93 92 91 90 89 87 86 85
74 | 99 98 96 95 94 93 92 91 90 89 88 87 85
76 | 99 98 97 96 95 93 92 91 90 89 88 87 86
78 | 99 98 97 96 95 94 93 92 91 90 89 88 87
80 | 99 98 97 96 95 94 93 92 91 90 89 88 87
======+=================================================================
Dry Bulb Reading.
| DIFFERENCE BETWEEN THE READINGS
| OF THE DRY AND WET BULB THERMOMETERS.
| ° ° ° ° ° ° ° ° ° ° ° ° °
| 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2
+=================================================================
° |
20 | 34 32 29 27 25 23 21 19 18 17 16 15 14
22 | 38 36 33 31 29 27 25 23 21 19 18 17 15
24 | 42 39 36 34 32 30 29 27 25 23 21 19 18
26 | 45 43 41 39 37 35 33 31 29 27 26 25 23
28 | 52 50 48 46 44 42 40 38 36 34 33 32 30
30 | 60 58 55 53 51 49 47 45 44 42 40 39 37
32 | 67 65 63 61 60 58 56 54 53 51 50 48 47
34 | 72 71 69 67 65 63 62 60 59 57 56 55 53
36 | 76 74 72 71 69 68 66 64 63 61 60 59 58
38 | 77 76 74 73 71 70 69 67 66 64 63 62 61
40 | 78 76 75 74 72 71 70 68 67 65 64 63 62
42 | 78 77 76 75 73 72 70 69 68 66 65 64 63
44 | 79 78 77 76 74 73 72 71 70 68 67 65 64
46 | 79 78 77 76 74 73 72 71 70 68 67 66 65
48 | 80 79 78 77 75 74 73 72 71 69 68 67 66
50 | 80 79 78 77 75 74 73 72 71 70 69 68 66
52 | 81 80 79 78 76 75 74 73 72 71 70 69 67
54 | 81 80 79 78 77 76 75 73 72 71 70 69 68
56 | 82 81 80 79 77 76 75 74 73 72 71 70 69
58 | 83 82 81 80 78 77 76 75 74 73 72 71 70
60 | 83 82 81 80 78 77 76 75 74 73 72 71 70
62 | 83 82 81 80 79 78 77 76 75 74 73 72 71
64 | 83 82 81 80 79 78 77 76 75 74 73 72 71
66 | 84 83 82 81 80 79 78 77 76 75 74 73 72
68 | 84 83 82 81 80 79 78 77 76 75 74 73 72
70 | 84 83 82 81 80 79 78 77 76 75 74 74 73
72 | 84 83 82 81 80 79 79 78 77 76 75 74 73
74 | 85 84 83 82 81 80 79 78 77 76 75 74 73
76 | 85 84 83 82 81 80 80 79 78 77 76 75 74
78 | 86 85 84 83 82 81 80 79 78 77 76 75 74
80 | 86 85 84 83 82 81 80 79 78 77 76 76 75
======+=================================================================
Dry Bulb Reading.
| DIFFERENCE BETWEEN THE READINGS
| OF THE DRY AND WET BULB THERMOMETERS.
| ° ° ° ° ° ° ° ° ° ° ° ° °
| 5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8 7.0 7.2 7.4 7.6 7.8
+=================================================================
° |
20 | 13 12 11 10 9 8 7 6 6 6 5 5 5
22 | 14 13 12 11 10 9 9 8 8 7 7 6 6
24 | 17 16 15 14 13 12 11 10 10 9 9 8 8
26 | 21 20 19 18 17 16 15 14 14 13 12 11 10
28 | 29 27 26 25 24 23 21 20 19 18 17 16 15
30 | 36 35 33 32 31 30 29 28 28 27 26 25 24
32 | 45 44 42 41 39 38 37 36 35 34 33 32 31
34 | 52 51 50 49 47 46 45 44 43 41 40 39 38
36 | 57 55 54 53 52 51 49 48 47 46 45 44 43
38 | 60 58 57 56 55 54 52 51 50 49 48 47 46
40 | 61 60 59 58 57 56 54 53 52 51 50 49 48
42 | 62 61 60 59 58 57 56 55 54 53 52 51 50
44 | 63 62 61 60 59 58 57 56 55 54 53 52 51
46 | 64 63 62 61 60 59 58 57 56 55 54 53 52
48 | 65 64 63 62 61 60 59 58 57 56 55 54 53
50 | 65 64 63 62 61 60 59 58 58 57 56 55 54
52 | 66 65 64 63 62 61 60 59 59 58 57 56 55
54 | 67 66 65 64 63 62 61 60 59 58 57 56 55
56 | 68 67 66 65 64 63 62 61 60 59 58 57 56
58 | 69 68 67 66 65 64 63 62 61 60 59 58 57
60 | 69 68 67 66 65 64 63 62 62 61 60 59 59
62 | 70 69 68 67 66 65 64 63 63 62 61 60 59
64 | 70 69 68 68 67 66 65 64 63 62 61 60 59
66 | 71 70 69 68 67 66 65 64 64 63 62 61 60
68 | 71 70 69 69 68 67 66 65 65 64 63 62 61
70 | 72 71 70 69 68 67 66 65 65 64 63 62 61
72 | 72 71 70 70 69 68 67 66 65 64 63 62 61
74 | 72 71 70 70 69 68 67 66 66 65 64 63 62
76 | 73 72 71 71 70 69 68 67 66 65 65 64 63
78 | 73 72 71 71 70 69 68 67 67 66 65 64 63
80 | 74 73 72 72 71 70 69 68 67 67 66 65 64
======+=================================================================
Dry Bulb Reading.
| DIFFERENCE BETWEEN THE READINGS
| OF THE DRY AND WET BULB THERMOMETERS.
| ° ° ° ° ° ° ° ° ° ° ° ° °
| 8.0 8.2 8.4 8.6 8.8 9.0 9.2 9.4 9.6 9.8 10.0 10.2 10.4
+=================================================================
° |
20 | 5 .. .. .. .. .. .. .. .. .. .. .. ..
22 | 5 5 5 5 4 4 .. .. .. .. .. .. ..
24 | 7 7 6 6 6 5 5 5 5 5 4 .. ..
26 | 10 9 9 8 8 7 7 6 6 6 5 5 5
28 | 15 14 14 13 13 12 12 11 10 10 9 9 8
30 | 23 22 21 20 19 18 17 16 15 15 14 14 13
32 | 30 29 29 28 27 27 26 25 24 23 23 22 21
34 | 37 36 36 35 34 33 33 32 31 30 30 29 28
36 | 42 41 40 39 39 38 37 36 35 34 34 33 32
38 | 45 44 43 42 42 41 40 39 38 37 36 35 34
40 | 47 46 45 44 44 43 42 41 40 39 38 37 36
42 | 49 48 47 46 45 44 43 42 41 40 40 39 38
44 | 50 49 48 47 46 46 45 44 43 42 41 41 40
46 | 51 50 49 48 47 47 46 45 44 43 43 42 41
48 | 52 51 50 49 48 48 47 46 45 44 44 43 42
50 | 53 52 51 50 49 49 48 47 46 45 45 44 43
52 | 54 53 52 51 50 50 49 48 47 46 46 45 44
54 | 55 54 53 52 51 51 50 49 48 47 47 46 45
56 | 56 55 54 53 52 52 51 50 49 48 48 47 46
58 | 57 56 55 54 53 53 52 51 50 49 49 48 47
60 | 58 57 56 55 54 54 53 52 51 50 50 49 48
62 | 58 57 56 56 55 55 54 53 52 51 51 50 49
64 | 59 58 57 57 56 55 55 54 53 52 52 51 50
66 | 60 59 58 58 57 56 55 54 53 52 52 51 51
68 | 60 59 58 58 57 56 56 55 54 53 53 52 52
70 | 61 60 59 59 58 57 57 56 55 54 54 53 52
72 | 61 60 60 59 58 58 57 56 55 54 54 53 53
74 | 62 61 61 60 59 59 58 57 56 55 55 54 54
76 | 63 62 61 61 60 59 59 58 57 56 56 55 54
78 | 63 62 61 61 60 60 59 58 57 56 56 55 55
80 | 64 63 62 62 61 60 60 59 58 57 57 56 55
======+=================================================================
Dry Bulb Reading.
| DIFFERENCE BETWEEN THE READINGS
| OF THE DRY AND WET BULB THERMOMETERS.
| ° ° ° ° ° ° ° ° ° ° ° ° °
| 10.6 10.8 11.0 11.2 11.4 11.6 11.8 12.0 12.2 12.4 12.6 12.8 13.0
+=================================================================
° |
20 | .. .. .. .. .. .. .. .. .. .. .. .. ..
22 | .. .. .. .. .. .. .. .. .. .. .. .. ..
24 | .. .. .. .. .. .. .. .. .. .. .. .. ..
26 | 5 5 4 .. .. .. .. .. .. .. .. .. ..
28 | 8 7 7 7 6 6 6 5 5 5 5 5 4
30 | 13 12 12 12 11 11 10 10 10 9 9 9 8
32 | 20 20 19 19 18 17 17 16 16 15 15 15 14
34 | 27 27 26 26 25 24 24 23 23 22 21 21 20
36 | 31 31 30 30 29 28 28 27 27 26 25 25 24
38 | 33 33 32 32 31 30 30 29 29 28 27 27 26
40 | 35 35 34 34 33 32 32 31 31 30 29 29 28
42 | 37 36 36 35 34 33 32 32 31 31 30 30 29
44 | 39 39 38 37 36 35 34 34 33 33 32 32 31
46 | 40 39 39 38 37 36 35 35 34 34 33 33 32
48 | 41 40 40 39 38 37 36 36 35 35 34 34 33
50 | 42 41 41 40 40 39 39 38 37 36 35 35 34
52 | 44 43 43 42 41 40 40 39 38 38 37 37 36
54 | 45 44 44 43 42 41 41 40 39 39 38 38 37
56 | 46 45 45 44 43 42 42 41 40 40 39 39 38
58 | 47 46 46 45 45 44 44 43 42 41 40 40 39
60 | 47 46 46 45 45 44 44 43 42 42 41 41 40
62 | 48 47 47 46 46 45 45 44 43 43 42 42 41
64 | 49 48 48 47 47 46 46 45 44 44 43 43 42
66 | 50 49 49 48 48 47 47 46 45 45 44 44 43
68 | 51 50 50 49 48 47 47 46 45 45 44 44 43
70 | 51 50 50 49 49 48 48 47 46 46 45 45 44
72 | 52 51 51 50 50 49 49 48 47 47 46 46 45
74 | 53 52 52 51 50 49 49 48 47 47 46 46 45
76 | 53 52 52 51 51 50 50 49 48 48 47 47 46
78 | 54 53 53 52 52 51 51 50 49 49 48 48 47
80 | 54 53 53 52 52 51 51 50 49 49 48 48 47
======+=================================================================
Dry Bulb Reading.
| DIFFERENCE BETWEEN THE READINGS
| OF THE DRY AND WET BULB THERMOMETERS.
| ° ° ° ° ° ° ° ° ° ° ° ° °
| 13.2 13.4 13.6 13.8 14.0 14.2 14.4 14.6 14.8 15.0 15.2 15.4 15.6
+=================================================================
° |
20 | .. .. .. .. .. .. .. .. .. .. .. .. ..
22 | .. .. .. .. .. .. .. .. .. .. .. .. ..
24 | .. .. .. .. .. .. .. .. .. .. .. .. ..
26 | .. .. .. .. .. .. .. .. .. .. .. .. ..
28 | .. .. .. .. .. .. .. .. .. .. .. .. ..
30 | 8 7 7 7 6 6 6 6 5 5 5 5 5
32 | 14 13 13 13 12 12 11 11 11 10 10 10 10
34 | 20 19 18 18 17 17 16 16 15 15 15 14 14
36 | 24 23 23 23 22 22 21 21 20 20 19 19 18
38 | 26 25 25 24 24 23 23 22 22 21 21 20 20
40 | 28 27 27 26 25 25 25 24 24 23 23 22 22
42 | 29 28 28 27 26 26 26 25 25 24 24 23 23
44 | 31 30 30 29 28 28 27 26 26 25 25 24 24
46 | 32 31 31 30 29 29 28 27 27 26 26 25 25
48 | 33 32 32 31 30 30 29 29 29 28 27 27 26
50 | 34 33 33 32 32 31 31 30 30 29 28 28 27
52 | 36 35 35 34 33 32 32 31 31 30 29 29 28
54 | 37 36 36 35 34 33 33 32 32 31 30 30 29
56 | 37 36 36 35 35 34 34 33 33 33 32 32 31
58 | 39 38 38 37 36 35 35 34 34 34 33 33 32
60 | 39 38 38 37 37 36 36 35 35 35 34 34 33
62 | 40 39 39 38 38 37 37 36 36 35 35 34 34
64 | 41 40 40 39 39 38 38 37 37 36 36 35 35
66 | 42 41 41 40 40 39 39 38 38 37 36 36 35
68 | 42 41 41 40 40 39 39 38 38 38 37 37 36
70 | 43 42 42 41 41 40 40 39 39 38 38 37 37
72 | 44 43 43 42 42 41 41 40 40 39 39 38 38
74 | 45 44 44 43 43 42 42 41 41 40 40 39 39
76 | 46 45 45 44 43 43 43 42 42 41 40 40 39
78 | 47 46 46 45 44 43 43 42 42 41 41 40 40
80 | 47 46 46 45 45 44 44 43 43 42 41 41 40
======+=================================================================
Dry Bulb Reading.
| DIFFERENCE BETWEEN THE READINGS
| OF THE DRY AND WET BULB THERMOMETERS.
| ° ° ° ° ° ° ° ° ° ° ° °
| 15.8 16.0 16.2 16.4 16.6 16.8 17.0 17.2 17.4 17.6 17.8 18.0
+=================================================================
° |
20 | .. .. .. .. .. .. .. .. .. .. .. ..
22 | .. .. .. .. .. .. .. .. .. .. .. ..
24 | .. .. .. .. .. .. .. .. .. .. .. ..
26 | .. .. .. .. .. .. .. .. .. .. .. ..
28 | .. .. .. .. .. .. .. .. .. .. .. ..
30 | 4 4 .. .. .. .. .. .. .. .. .. ..
32 | 9 9 9 8 8 8 7 7 7 7 6 6
34 | 14 13 13 13 12 12 12 11 11 11 10 10
36 | 18 17 17 16 16 15 15 14 14 14 13 13
38 | 20 19 19 18 18 17 17 16 16 16 15 15
40 | 22 21 21 20 20 19 19 18 18 18 17 17
42 | 23 22 22 21 21 20 20 19 19 19 18 18
44 | 24 23 23 22 22 21 21 20 20 20 19 19
46 | 25 24 24 23 23 22 21 21 21 21 20 20
48 | 26 25 24 23 23 22 22 22 21 21 20 20
50 | 27 26 26 25 25 24 24 23 22 22 21 21
52 | 28 27 27 26 26 25 25 24 24 24 23 23
54 | 29 28 28 27 27 26 26 25 25 25 24 24
56 | 31 30 29 29 28 28 27 27 26 26 25 25
58 | 32 31 31 30 30 29 29 28 28 28 27 27
60 | 33 32 32 31 31 30 30 29 29 28 28 27
62 | 33 33 33 32 32 31 31 30 30 29 29 28
64 | 34 34 33 32 32 31 31 30 30 29 29 29
66 | 35 34 34 33 33 32 32 32 31 31 30 30
68 | 36 35 35 34 34 33 33 33 32 32 31 31
70 | 36 36 35 35 35 34 34 33 33 32 32 31
72 | 37 37 36 36 35 35 34 34 34 33 33 32
74 | 38 38 37 37 36 36 35 35 35 34 34 33
76 | 39 38 38 37 37 37 36 36 36 35 35 34
78 | 39 39 39 38 38 37 37 36 36 35 35 34
80 | 40 39 39 38 38 37 37 36 36 35 35 35
+=================================================================
TABLE SHOWING THE PRESSURE OF SATURATED AQUEOUS VAPOUR IN INCHES OF
MERCURY AT LATITUDE 45° FOR EACH DEGREE FAHRENHEIT FROM -30° TO 119°.
-----+--------+----+--------+----+--------+----+--------+-----+--------
° | Inch. | ° | Inch. | ° | Inch. | ° | Inch. | ° | Inches.
-30 | 0.0099 | 0 | 0.0440 | 30 | 0.1665 | 60 | 0.5192 | 90 | 1.4128
-29 | 0.0105 | 1 | 0.0461 | 31 | 0.1738 | 61 | 0.5379 | 91 | 1.4578
-28 | 0.0111 | 2 | 0.0482 | 32 | 0.1815 | 62 | 0.5572 | 92 | 1.5040
-27 | 0.0117 | 3 | 0.0504 | 33 | 0.1888 | 63 | 0.5771 | 93 | 1.5514
-26 | 0.0123 | 4 | 0.0527 | 34 | 0.1964 | 64 | 0.5976 | 94 | 1.6001
-25 | 0.0130 | 5 | 0.0551 | 35 | 0.2043 | 65 | 0.6187 | 95 | 1.6502
-24 | 0.0137 | 6 | 0.0577 | 36 | 0.2125 | 66 | 0.6405 | 96 | 1.7017
-23 | 0.0144 | 7 | 0.0604 | 37 | 0.2210 | 67 | 0.6630 | 97 | 1.7546
-22 | 0.0152 | 8 | 0.0632 | 38 | 0.2297 | 68 | 0.6862 | 98 | 1.8088
-21 | 0.0160 | 9 | 0.0661 | 39 | 0.2388 | 69 | 0.7101 | 99 | 1.8646
-20 | 0.0168 | 10 | 0.0691 | 40 | 0.2482 | 70 | 0.7347 | 100 | 1.922
-19 | 0.0177 | 11 | 0.0723 | 41 | 0.2579 | 71 | 0.7601 | 101 | 1.980
-18 | 0.0186 | 12 | 0.0756 | 42 | 0.2679 | 72 | 0.7862 | 102 | 2.041
-17 | 0.0196 | 13 | 0.0790 | 43 | 0.2783 | 73 | 0.8131 | 103 | 2.103
-16 | 0.0206 | 14 | 0.0825 | 44 | 0.2890 | 74 | 0.8409 | 104 | 2.166
-15 | 0.0217 | 15 | 0.0862 | 45 | 0.3001 | 75 | 0.8695 | 105 | 2.231
-14 | 0.0228 | 16 | 0.0901 | 46 | 0.3116 | 76 | 0.8989 | 106 | 2.298
-13 | 0.0239 | 17 | 0.0942 | 47 | 0.3235 | 77 | 0.9292 | 107 | 2.366
-12 | 0.0251 | 18 | 0.0985 | 48 | 0.3358 | 78 | 0.9604 | 108 | 2.437
-11 | 0.0263 | 19 | 0.1030 | 49 | 0.3485 | 79 | 0.9925 | 109 | 2.509
-10 | 0.0276 | 20 | 0.1076 | 50 | 0.3616 | 80 | 1.0255 | 110 | 2.583
- 9 | 0.0289 | 21 | 0.1124 | 51 | 0.3751 | 81 | 1.0595 | 111 | 2.659
- 8 | 0.0303 | 22 | 0.1174 | 52 | 0.3891 | 82 | 1.0945 | 112 | 2.736
- 7 | 0.0318 | 23 | 0.1226 | 53 | 0.4036 | 83 | 1.1305 | 113 | 2.817
- 6 | 0.0333 | 24 | 0.1282 | 54 | 0.4186 | 84 | 1.1675 | 114 | 2.898
- 5 | 0.0349 | 25 | 0.1339 | 55 | 0.4341 | 85 | 1.2056 | 115 | 2.982
- 4 | 0.0366 | 26 | 0.1399 | 56 | 0.4501 | 86 | 1.2447 | 116 | 3.067
- 3 | 0.0383 | 27 | 0.1461 | 57 | 0.4666 | 87 | 1.2850 | 117 | 3.156
- 2 | 0.0401 | 28 | 0.1526 | 58 | 0.4836 | 88 | 1.3264 | 118 | 3.246
- 1 | 0.0420 | 29 | 0.1594 | 59 | 0.5011 | 89 | 1.3690 | 119 | 3.338
-----+--------+----+--------+----+--------+----+--------+-----+--------
[Illustration: ISOTHERMAL LINES SHOWING THE MEAN TEMPERATURE (FAHR.) OF
THE GLOBE FOR JANUARY.
_Published by the Royal Geographical Society in “Hints to Travellers.”
1921._]
[Illustration: ISOTHERMAL LINES SHOWING THE MEAN TEMPERATURE (FAHR.) OF
THE GLOBE FOR JULY.
_Published by the Royal Geographical Society in “Hints to Travellers.”
1921._]
[Illustration: ISOBARIC LINES AND PREVAILING WINDS OF THE GLOBE FOR
JANUARY.
_Published by the Royal Geographical Society in “Hints to Travellers.”
1921._]
[Illustration: ISOBARIC LINES AND PREVAILING WINDS OF THE GLOBE FOR JULY.
_Published by the Royal Geographical Society in “Hints to Travellers.”
1921._]
[Illustration: MEAN ANNUAL RAINFALL OF THE GLOBE.
_Published by the Royal Geographical Society in “Hints to Travellers.”
1921._]
II.
PHOTOGRAPHY.
_By J. THOMSON, formerly Instructor in Photography R.G.S._
Revised by J. MCINTOSH, _Secretary of the Royal Photographic Society of
Great Britain._
The photographic camera should form an essential part of the traveller’s
outfit, as it affords the only trustworthy means of obtaining pictorial
records of his journey, and it is also helpful in making the survey of
a new region, delineating its contours, its geological and botanical
features, and ethnographical types of race. The camera and materials
necessary for a journey may be readily obtained, so designed as to
minimise space and weight, and in every way so perfectly adapted to
the traveller’s needs as to ensure successful results in every variety
of climate, and render the operation of taking a photograph extremely
simple. It is necessary, however, that the traveller should make himself
master of the principles involved in the production of a successful
photograph, as he will have to depend on his personal effort in exposing
and developing the plate, etc. He should also acquire a knowledge of the
construction of the camera, to enable him to effect slight repairs when
necessary.
In selecting an outfit he must first decide upon the size of plate to be
carried, and that need not exceed what is termed “half-plate,” 6½ × 4¾
inches; which is large enough for the best work. The smallest effective
size for scientific work may be “quarter-plate,” 4¾ × 3¼ inches; in
use in many hand-cameras. Negatives on this scale, if perfect, may
be enlarged for book illustration, or printed as lantern slides. The
two sizes given form a very complete outfit when extra weight may be
conveniently carried.
_Selecting a Camera._—The cameras should have bellows bodies of Russian
leather which folds into small space, the woodwork must be well seasoned
to prevent warping, or cracking under a hot sun. The framework should be
metal-bound at the corners, and the camera fitted with a rising front and
swing back, although the swing back is not indispensable. A reversible
back (now universal) is of advantage, as it enables the operator to
take vertical or horizontal views without turning the camera on its
side. If the camera is fitted with a swing-front, the swing-back becomes
unnecessary. The swing-front is in all ways preferable to the swing-back.
[Illustration: BELLOWS CAMERA.]
_The Hand-camera._—Hand-cameras are designed to carry a dozen or more
plates or films in flat sheets or in spools, so arranged inside the
camera as to be changed after each exposure by simply turning a milled
head, or moving a lever. Rolled films are not recommended for travellers’
use in hot and humid climates. Sir Martin Conway says: “A traveller who
carries glass plates and flat films will probably bring home a larger
percentage of good negatives from a long mountain journey than one who
relies upon spools of films.” There can be no question about the force
and accuracy of this statement. My own experience goes to prove that a
camera arranged for glass plates and flat films is best.
[Illustration: TWIN-LENS CAMERA (OPEN).]
The twin-lens hand-camera made by Ross, of New Bond Street, London,
offers several advantages in its design. It is fitted with a focal-plain
shutter which is in every way simple and effective. The twin lenses are
of equal focal length, enabling the object to be photographed to be seen
on the same scale as it will appear in the finished negative, so that
just what is required may be embraced in the field. It consists of a
stout body of thoroughly seasoned hardwood, not easily damaged by rough
usage. The principal fittings required for adjusting the instrument are
inside, protected by the outer case. The exception to this arrangement
is the milled head by means of which the two lenses are focussed at the
same time. The lenses are of uniform focal length, so that the image
transmitted by the “finder” is a counterpart of the image which falls
upon the sensitive plate. The advantage of this is that the object to be
taken is seen on the finder-screen to scale exactly as it will appear
in the finished photograph. By this means the operator has it in his
power to place the object in the required position on the screen at the
moment of exposure. This is of signal importance if the object is moving
about. It must also be noted that this form of camera may be used when
the operator is facing at right angles to the object to be photographed.
I have frequently found that natives of foreign countries resent the
liberty taken of pointing a camera at them, and fly as if they expected
to be shot. The slides are each made to hold two plates, or flat films.
They are strong, serviceable, and easily managed, while the body of the
camera is so arranged as to carry a roll holder. The camera can also be
adapted to stereoscopic work, and fitted for the use of glass plates,
flat films, or rolled films.
A light tripod stand should be taken for supporting the camera when
longer exposures are required than can be given in the hand. A very
satisfactory compromise has been adopted by Sir W. Abney between using
the camera in the hand and on a tripod. He rests the camera on top of
a walking-stick when making hand exposures, with the result that he
overcomes all tremor caused by pulsation, and so secures photographs full
of sharp detail.
The same object is obtained by the reflex type of camera in which the
image formed by the lens is thrown upon a silver-faced mirror and
reflected upward to the focussing screen. Focussing can be carried out,
and a moving object followed till the moment when the exposure is made.
The reflex type of camera is a trifle less bulky than the twin-lens, and,
of course, only one high class and expensive lens is required.
_A Focussing-cloth._—This is used for keeping out the light while
focussing, being thrown over the camera and the head of the operator.
It is generally made of black velvet, but waterproof sheeting is much
better. It should have rings sewn on to one edge, or some arrangement by
which it may be attached to the camera so as not to be blown away.
_Camera-stand._—There are many varieties of tripod stands, with legs
either folding or sliding into a small compass. For mountainous country
it is of great advantage to have a stand with telescopic legs, as they
can be readily altered in length so as to stand firmly on slopes or rocky
ground. The smallest size, weighing about 3 lbs., and measuring 33 in.
long when closed, and standing about 4 ft. 6 in. high, is steady enough
to support a 6½ × 4¾ camera without perceptible vibration in a moderate
wind.
_A Small Circular Cup Level_, let into the wood of the camera, for
levelling the camera on the tripod, is a useful addition.
_Lenses._—There are many lenses in the market, and as it is impossible
to do good work with an inferior lens, it is necessary to exercise great
care in selecting this part of a photographic outfit. Lenses known as
rectilinear or symmetrical are useful to a scientific explorer, and are
well fitted for producing pictorial effect in his work.
Ordinary portrait lenses are designed specially for rapid work, and
this is attained at the cost of qualities in a lens most useful to an
explorer. The so-called portrait combination should therefore be avoided.
Rectilinear and symmetrical lenses give true images of objects to be
photographed free from distortion, so that straight lines are reproduced
as straight lines. In this way they are invaluable where accurate
measurements have to be taken from photographs produced by them.
[Illustration: HOMOCENTRIC LENS.]
Ross’s homocentric lens is one of the most useful lenses. It has a flat
field, is free from what is called “coma” and astigmatism, and is so
perfectly corrected as to fit it for interior and exterior work alike. It
is also a rapid hand-camera lens. The homocentric are made in series to
suit all cameras. Other lenses may also be noted, viz.: _c._ Dallmeyer’s
rapid rectilinear, including about 37°. _d._ Zeiss’s anastigmat, made
by Ross, consists of a double front lens and a triple back lens. It
is intended for portraits, groups, copying, and general outdoor work.
The combinations being brought closely together, gives them great
illuminating power. They have an angular aperture of from 858 to 908, and
can therefore be used as wide-angle lenses when desired. In consequence
of the peculiar system of correction for oblique pencils adopted in these
lenses, they behave somewhat differently from the usual types with regard
to the mode of compensating the effect of the resulting aberrations
between centre and margin of the field. This is, of course, only possible
in the case of perfectly plane objects. In all other cases—landscape,
instantaneous work, or interiors—the centre should be focussed rather
than objects at a distance or foreground.
_Focus._—It will be sufficient to say that focal length means the
distance between the diaphragm of a lens, and the ground-glass screen
when the image of a distant object is seen most distinctly on the screen.
_Exposure Tables._—Exposure tables are based on the focal length of a
lens, in relation to the diameter of the diaphragm of a lens. Thus,
if the focus is eight inches and diameter of diaphragm one inch, the
relationship will be expressed by _f_/8 or by the uniform standard
number 4, and so on, as in table. The diaphragms are so arranged for
size of opening that each succeeding number requires double the exposure
necessary with the one preceding it. _f_/11.3, for example, requires
double the exposure wanted with _f_/8.
U. S. Nos. 4 8 16 32 64 128 256
_f_ _f_ _f_ _f_ _f_ _f_ _f_
Ratio of Stops. --- ---- --- ---- --- --- ---
8 11.3 16 22.6 32 45.2 64
Such tables are useful guides to the relative duration of exposure with
diaphragms of different sizes applied to the same lens. They afford no
clue, however, to time of exposure to be given with any particular lens
or diaphragm. This can be best ascertained by experience, as duration of
exposure of a plate or film in the camera depends on the sensitiveness
of the plate, the time of day, the sun, the state of the atmosphere,
the nearness or distance of the object to be photographed, etc. To take
an extreme case of the difference of time required to impress the plate
with the image of an exterior view and that of an interior, a landscape
open and well lighted may be taken in the fraction of a second, while a
dimly-lighted interior with the same lens would require an hour, both
being taken with plates coated with the same emulsion. The duration of
exposure may be approximately estimated by using an exposure meter such
as may be obtained from any photographic dealer’s. It is useful to keep
an exposure record; a handy book for this purpose is published by Messrs.
Burroughs & Welcome. In this book rules are set down for exposure during
different months of the year and for different latitudes. These are apt
to prove misleading to the amateur. The simplest method of measuring
the actinic power of light in any latitude, and at any moment, is by
actinometer, giving plate-speeds, focus of lens, etc. Watkins “Bee
Meters” are excellent for this purpose.
_Sensitive Plates or Films._—Gelatine plates are now made commercially by
a large number of firms and of great excellence; they keep indefinitely
before exposure, and for a long time afterwards and before development
and under some circumstances (as for instantaneous pictures, portraits,
and dimly-lighted interiors) will give results which could hardly be
obtained at all on collodion plates. Gelatine plates are made of various
degrees of sensitiveness; the slowest are best for ordinary landscape
work. They are generally supplied in parcels of a dozen each, packed face
to face with strips of folded paper between opposite edges. The card
boxes in which they are usually packed are an insufficient protection
against injury and damp. In all cases it is advisable, and for sea
voyages and damp climates essential, to have each package of a dozen
plates soldered down in a tin case, and afterwards packed in a light
wooden box with tow or cotton wool, and the box screwed (not nailed)
down. In packing them up again after exposure or after development, a
good plan (due to Sir W. Abney) is to provide oneself with a number of
cardboard frames exactly the size of the plates, made of strips of card
about ¼ in. wide, one of which is inserted between every two plates film
to film. The packages thus made up should be soldered down again, and
treated with at least as much care as the original plates in packing.
Should there be no available means of resoldering the boxes, it will be
better to have tin boxes with the lid turned well down, the joinings to
be closed by strong well-gummed paper or medical rubber strapping. It
will also be well to be provided with a supply of waterproof paper, or
cloth, as an additional precaution in packing and in case of emergencies.
Sensitive films in rolls or spools are made by the Kodak and other
companies, and may be used successfully in their proper roll-holders
when they can be kept perfectly dry in temperate climates. Flat films
made by Kodak, Ltd., and others have many advantages for travellers. The
celluloid of which they are made is very much lighter than glass, and
in exposure and development may be treated in the same way as a glass
plate. When plates can be carried, the extra weight is compensated for by
greater certainty of success, and general excellence in the photographs.
_How to Keep Plates and Films Dry._—When the traveller has a long journey
before him, and the prospect of storing his plates and films for months
both before and after exposure, it is of the greatest importance that
precautions should be taken against the inroads of damp. This applies
with full force when the country to be explored has a hot, humid climate.
Plates and films that have absorbed moisture, causing decomposition in
the sensitive gelatine coating, are frequently brought back to this
country to be developed, and are the most fruitful cause of failure. The
remedy is simple, but can only be applied when packing and repacking
the plates. Some guarantee should be sought from makers of plates and
films that they are packed perfectly dry, and that the packing used is
also dry. Assuming that work has to be done in a damp climate and that
the plates have been exposed in the camera and require to be repacked,
they should be dried in a box containing a small quantity of chloride
of calcium. The box used for drying may be also designed to carry the
camera and outfit. It should have a lid with a rim of rubber padding, so
that by putting the lid on and a weight on it, the box would be fairly
air-tight. Stack the exposed plates, or films, in the bottom of box, so
separated as to permit the passage of air between. Place a cup or saucer
on the bottom of box containing chloride of calcium. (The chloride should
be first dried on a piece of iron over a fire.) Put on the lid and allow
the plates to remain for an hour or more. Dry all the packing materials,
remove the plates from the box and repack. The chloride will have
absorbed the moisture in the plates, and rendered them quite dry and safe
for preserving for an indefinite length of time.