zone where it _never_ falls, one locality in Norway where it falls
three days out of four, and another on the western side of Patagonia,
at the base of the Andes, where it falls every day. The quantities
recorded as having fallen at one time in some localities are simply
appalling. A fall of one inch is considered a very heavy rain in Great
Britain, and this fact will enable the reader partially to realize the
following stupendous recorded falls:—Loch Awe, Scotland, 7 inches in
30 hours; Joyeuse, France, 31 inches in 22 hours; Gibraltar, 33 inches
in 26 hours; hills above Bombay, 24 inches in one night; and on the
Khasia Hills, where the annual rainfall is 600 inches, 30 inches have
been known to fall on each of five successive days. Mr. G. J. Symons,
the able editor of the “Meteorological Magazine,” and indefatigable
superintendent of 2,000 Rain Gauges throughout the United Kingdom, has
compiled a table, showing the equivalents of rain in inches, its weight
per acre, and bulk in gallons, the following portion of which, while
very useful to the farmer, will enable the curious reader to make some
interesting calculations, based on the figures quoted above:—
TABLE SHOWING EQUIVALENT OF INCHES OF RAIN IN GALLONS,
AND WEIGHT PER ACRE.
Inches of Rain Tons per Acre Gallons per Acre
0·1 10 2262
0·2 20 4525
0·3 30 6787
0·4 40 9049
0·5 50 11312
0·6 61 13574
0·7 71 15836
0·8 81 18098
0·9 91 20361
1·in. 101 22623
The instruments called Rain Gauges or Pluviometers are, as their name
implies, constructed to measure the amount of rain falling in any given
locality, and those in most general use have this principle in common:
that the graduated glass always bears a definite relation to the area
of the receiving surface. A very extraordinary and hitherto unexplained
fact in connection with the fall of rain, and which justifies the
opinion that its formation is not limited to the region of visible
cloud, is that a series of rain gauges placed at different elevations
above the soil are found to collect very different quantities of rain,
the amount being _greater_ at the _lower_ level. Thus, twelve months’
observations by Dr. Heberden determined that the amount of rain on the
top of Westminster Abbey was only twelve inches, that on a house close
by but much lower eighteen inches, and on the ground during the same
interval of time twenty-two inches. Accordingly, ten inches is the
height at which meteorologists have agreed the edge of the rain gauge
should be placed from the ground. The spot chosen should be perfectly
level, and at least as far distant from any building or tree as the
building or tree is high, and, if the gauge cannot be equally exposed
to all points, a south-west aspect is preferable. It is also important
that the rain gauge should be well supported, in order to avoid its
being blown over by the wind; and, should frost follow a fall of rain,
the instrument should be conveyed to a warm room to thaw before
measuring the collected contents. The graduated glass furnished with
each instrument should stand quite level when measuring the rain, and
the reading be taken midway between the two apparent surfaces of the
water.
The best form of rain gauge is that in use in the Meteorological Office.
[Illustration: 48.
Howard’s Rain Gauge.
Scale about 1/5.]
Howard’s Rain Gauge consists of a vertical glass receiver, or bottle,
through the neck of which the long terminal tube of a circular funnel,
five inches in diameter, is inserted. A metal collar or tube fits over
the outside of the neck of the receiver, and aids in keeping the funnel
level, while the tube extends to within half an inch of the bottom,
thus ensuring the retention of every drop of rain which falls within
the area of the funnel. The glass vessel furnished with the instrument
is graduated to 100ths of an inch. A modification of this instrument is
made with a glass tube at the side graduated to inches, 10ths, and
100ths, showing the amount of rainfall by direct observation, thus
dispensing with the use of a supplementary graduated measure.
In Glashier’s Rain Gauge special provision is made, in two ways, to
prevent possible loss by evaporation, even in the warmest months of the
year. 1. The receiving vessel is partly sunk beneath the soil, thus
keeping the contents cool. 2. The receiving surface of the funnel,
accurately turned to a diameter of eight inches, terminates at its
lower extremity in a curved tube, which, by always retaining the last
few drops of rain, prevents evaporation. The graduated vessel, in this
instance also, is divided to 100ths of an inch, having due regard to
the larger area, 8 in. of the funnel. For use in tropical climates,
where, as has been shown, the rainfall is excessive, a modification of
this instrument is supplied by the instrument makers, having an extra
large receiver and tap for drawing off the collected rain.
Luke Howard, in his “Climate of London,” says: “It must be a subject of
great satisfaction and confidence to the husbandman to know at the
beginning of a summer, by the certain evidence of meteorological
results on record, that the season, in the ordinary course of things,
may be expected to be a dry and warm one, or to find, in a certain
period of it, that the average quantity of rain to be expected for the
month has fallen. On the other hand, when there is reason, from the
same source of information, to expect much rain, the man who has
courage to begin his operations under an unfavourable sky, but with
good ground to conclude, from the state of his instruments and his
collateral knowledge, that a fair interval is approaching, may often be
profiting by his observations, while his cautious neighbour, who
‘waited for the weather to settle,’ may find that he has let the
opportunity go by.” This superiority, however, is attainable by a very
moderate share of application to the subject, and by the keeping of a
plain diary of the barometer and rain gauge, with the hygrometer and
vane under his daily notice.
[Illustration: 49.
Symons’s Rain Gauge.
Scale about 1/7.]
Symons’s Rain Gauge resembles Howard’s, but has the advantage of having
the glass receiver enclosed in a black or white japanned metal or
copper jacket with openings permitting an approximate observation of
the collected rain. The metal jacket is also furnished with strong iron
spikes, which are firmly pressed into the soil, as shown at Fig. 49,
thus ensuring perfect steadiness by its power to resist the wind. The
graduated measure contains half an inch of rain (for a 5 inch circle)
divided into 100ths.
[Illustration: 50.
Symons’s Storm Rain Gauge.
Scale about 1/12.]
Mr. Symons has devised another rain gauge of so ingenious and
interesting a character that it needs only to become generally known
among amateur meteorologists to be in universal demand. By its means an
observer at a distant window may read off the rain as it falls. It is
shown at Fig. 50, where the usual 5-inch funnel surmounts a long glass
tube attached to a black board bearing a very open scale marking tenths
of an inch in _white_ lines; a white float inside the tube constitutes
the index, which rises as the rain increases in quantity. If, as
sometimes happens during a thunderstorm, the rainfall is excessive, a
second tube on the left permits the measurement of a second inch of
rain. It will be obvious that if the _time_ at which the rain begins to
fall be noted the _rate_ at which it falls, as well as the quantity, is
indicated at sight by this instrument.
[Illustration: 51.
Beckley’s Pluviograph. Scale about 1/7.]
Crossley’s Registering Rain Gauge has a receiving surface of 100 square
inches. The rain falling within this area passes through a tube to a
vibrating bucket, which sets in motion a train of wheels, and these
move the indices on three dials, recording the amount of rain in
inches, 10ths, and 100ths. Printed directions are furnished with each
instrument, and the simplicity of the mechanism ensures due accuracy. A
test measure, holding exactly five cubic inches of water, sent with
each gauge, affords the means of checking its readings from time to
time.
Beckley’s Pluviograph possesses the exceptional merit of recording with
equal precision all rainfalls, from a slight summer shower to a heavy
storm of rain. It may be placed in a hole in the ground, with the
receiving surface raised the standard height of ten inches above its
level.
Fig. 51 illustrates the construction of the instrument.
[Illustration: 52. 53.
Stutter’s Self-recording Rain Gauge. Scale about 1/7.]
The funnel has a receiving surface of 100 square inches, protected by a
lip 1-1/4 inch deep, to retain the splashes. The rain flows into a
copper receiving vessel on the right, which, floating in a cistern of
mercury, sinks and draws down with it a pencil, which records the event
on a white porcelain cylinder moved by a clock. When the receiving
vessel is full the syphon comes into action, rapidly drawing off _the
whole_ of the water, the vessel rising almost at a bound, the action
being recorded by a vertical line on the porcelain cylinder. Two or
more cylinders are supplied with each instrument; and, as the pencil
marks are readily removed by a little soap and water, a clean one may
be always kept at hand for exchange once in every twenty-four hours.
The Rev. E. Stutter’s Self-recording Rain Gauge is ingenious, and for a
self-recording instrument is very moderate in price, while it
efficiently shows the rainfall for every hour in the twenty-four (Figs.
52, 53).
An eight-day clock with its upright spindle revolves a small funnel
with a sloping tube, the end of which passes successively over the
mouth of the twelve or twenty-four compartments in the rim of the
instrument; beneath each compartment is placed a tube, as shown in the
sectional figure. All rain received by the outer funnel drips into the
smaller revolving funnel, and flows down the sloping tube, the end of
which is timed to take an hour in passing over each compartment, so
that the rain, for example, which falls between twelve and one o’clock
will be found in the tube marked 1. Each tube can contain half an inch
of rain, and any overflow falls into a vessel beneath, and can be
measured; the tube which has overflown shows the hour.
V.—MOTION.
Wind is air in motion. The motion of the air is caused by inequality of
temperature. The earth becomes warmed by the sun, and radiates the heat
thus acquired back upon the air, which, expanding and becoming lighter,
ascends to higher regions, while colder and denser currents rush in to
occupy the vacated space. Two points are to be noted in connection with
this rush of air which we call wind, viz., its _direction_ and
_velocity_ or _force_. Both are estimated scientifically by instruments
called Anemometers,[13] while mariners and the dwellers on our coasts
have a nomenclature of their own by which to indicate variation in the
_force_ of the wind, founded on the amount of sail a vessel can carry
with safety at the time. In the matter of _direction_ winds are classed
as constant, periodical, and variable.
[Footnote 13: _Anemos_, the wind; _metron_, measure.]
CONSTANT WINDS.—_The Trade Winds._—The violent contrast between the
temperature of the equator and the poles is well known, and from the
vast area included within the tropics ascending currents of rarefied
air are incessantly rising and being as incessantly replaced by a rush
of cold air from the poles to the equator. Were the earth stationary,
this interchange would be of the simplest kind; on arriving within the
influence of the ascending equatorial current the air from the poles
would be carried to the higher regions and turning over would proceed
to the poles, and, becoming cold and dense in traversing the higher
stratum, would descend and resume its course _ad infinitum_. The
revolution of the earth on its axis changes all this: the first effect
is that the air at the equator is borne along with the earth at the
rate of seventeen miles a minute from west to east, a rate which
diminishes at 60° of latitude to one-half that velocity, until at the
poles it is nothing; consequently a _slow_ north wind flowing to the
equator is continually passing over places possessing a higher velocity
than itself, and not immediately acquiring that velocity, there is
according to the law of the composition of forces a compromise effected
resulting in a north-east wind. In a similar manner the same process in
the southern hemisphere results in a south-east wind. These winds have
acquired the name of _Trade Winds_ on account of the facilities
afforded to navigation by their constancy. The _North Trades_ occur in
the Atlantic between 9° and 30° and in the Pacific between 9° and 26°.
The _South Trades_ occur in the Atlantic between lat. 4° N. and 22° S.
and in the Pacific between latitude 4° N. and 23-1/2° S. These limits
extend northward with the sun from January to June, and southward from
July to December.
Parallel to the equator and extending between 2° and 3° on each side is
a broad belt, where the north and south trades neutralize each other,
producing what is called the “_Region or Belt of Calms_.” Though wind
is absent, thunderstorms and heavy rains are of daily occurrence.
When Humboldt ascended Teneriffe the trade wind was blowing at its base
in the usual direction, but on arriving at the summit he found a strong
wind blowing in the opposite direction. Observation has shown that this
upper current prevails north and south of the equator, and that, after
passing the limit of the trade winds, it descends to form the
south-_west_ winds of the north temperate zone and the north-_west_
winds of the south temperate zone; the _westing_ being due to the same
cause as the _easting_ in the regular trades, viz., the rotation of the
earth on its axis. These winds are called the Return Trades, but are
not equal in constancy to the regular trade winds.
PERIODICAL WINDS.—_Land and Sea Breezes_ occur on the coasts, chiefly
in tropical countries, but sometimes in Great Britain during the summer
months when the land during the day becomes very hot, causing an
ascending column of air, which is replaced by a comparatively colder
stream flowing inwards from the sea. At sunset the conditions are
reversed, the earth becomes rapidly cooled by radiation, the sea
continuing comparatively warm, the air over it ascends, and its place
is supplied by a cold breeze, which “blows off the shore,” as
illustrated by the diagrams and the following experiment—In the centre
of a large tub of water float a water plate containing hot water,
imagine the former to be the ocean and the latter the heated land,
rarefying the air over it. Light a candle and blow it out and hold it
while still smoking over the cold water, when the smoke will be seen to
move towards the plate. The reverse of this takes place if the tub be
filled with hot water and the water plate with cold. When this
phenomenon takes place on a large scale, as in the case of the north
trade winds being drawn from their course by the heated shores of
Southern Asia, the gigantic sea breeze thus produced is called the
south-west monsoon. This occurs from April to October, when the sun is
north of the equator. When the sun is south of the equator—that is,
from October to April—the analogue of the land breeze is produced, and
is called the north-east monsoon.
VARIABLE WINDS.—The character of this class of winds is determined by
the physical configuration of the country in which they occur. Some
tracts are marked by luxuriant vegetation, others are bare. Here
mountains lift their awful fronts and “midway leave the storm,” there
an arid plain extends itself to the seashore, or inland, towards a
chain of lakes. Within the tropics these purely local conditions are
insufficient to overcome the force of the prevalent atmospheric
currents: such, however, is not the case beyond the tropical zone.
There the variable winds prevail, for which space permits only the
mention of their names:—The _Simoom_ (from the Arabic _samma_, hot),
peculiar to the hot sandy deserts of Africa and Western Asia. The
_Sirocco_ blows over the two Sicilies as a hot wind from the south. It
extends sometimes to the shores of the Black and Caspian seas,
spreading death among animals and plants. The _Solano_ prevails at
certain seasons in the south of Spain: its direction is south-east. The
_Harmattan_ is another wind of the same class, peculiar to Senegambia
and Guinea. The _Puna Winds_ blow for four months over a barren
tableland called the Puna, in Peru. They are a portion of the
south-east trade winds, which, having crossed the Pampas, are thereby
deprived of moisture, and become the most parching wind in the world.
The _East Winds_, peculiar to the spring in Britain, blowing as they do
through Russia, over Europe, are a portion of the great polar current,
distinctive of that season of the year. They are dry and parching,
every one being familiar with the unpleasant bodily sensations
attendant on this much-abused and yet most beneficent wind.
The _Etesian Winds_ are drawn from the north across the Mediterranean
by the great heat of the African desert. The _Mistral_ is a strong
north-west wind peculiar to the south-east of France. The _Pampero_ is
a north-west wind, blowing in summer from the Pampas of Buenos Ayres.
As long ago as the year 1600 Lord Bacon remarked that the
preponderating tendency of the wind was decidedly to veer _with_ the
sun’s motion, thus passing from N. through N.E., E., S.E., to south,
thence through S.W., W.N.W., to N.; also, that it often makes a
complete circuit in that direction, or more than one in succession
(occupying sometimes many days in so doing), but that it rarely backs,
and very rarely or never makes a complete circuit in the contrary
direction. The merit of having first demonstrated that this tendency is
a direct consequence of the earth’s rotation is due to Professor Dove,
of Berlin, who has also shown that the three systems of atmospheric
currents just treated of, viz., the constant, periodical, and variable
winds, are all amenable to the same influence.
As to the _mode_ of observing the wind, Admiral Fitzroy recommends that
a true east and west line should be marked _about the time of the
equinox_, and the north, south, and other points of the compass being
added, to take the bearings of the wind in relation to a dial so
prepared, the indications of the _lower_ stratum of clouds in
conjunction with vanes and smoke being preferred to any other.
The direction of the wind should always be given according to _true_,
and not to _compass bearings_. Two points to the westward nearly
represents the amount of “Variation of the Compass” for the British
Isles, which yields the following table for the conversion of
directions observed by the compass in Great Britain and Ireland to
approximate true bearings.
+---------------------+-----+-----+-----+-----+-----+-----+-----+
|Compass bearings. N | NNE | NE | ENE | E | ESE | SE | SSE |
|True bearings. NNW| N | NNE | NE | ENE | E | ESE | SE |
+---------------------+-----+-----+-----+-----+-----+-----+-----+
|Compass bearings. S | SSW | SW | WSW | W | WNW | NW | NNW |
|True bearings. SSE| S | SSW | SW | WSW | W | WNW | NW |
+---------------------+-----+-----+-----+-----+-----+-----+-----+
“One may call a very simple diagram, a circle divided by a diameter
from north-east to south-west, the _thermometer compass_. While the
wind is shifting from south-west, by west, north-west, and _north to
north-east_, the thermometer is falling, but while shifting from
north-east, by east, south-east and south, towards _south and
south-west_, the thermometer is rising. Now the barometric column does
just the reverse. From north-east the barometer falls as the wind
shifts through the east to south-east, south, and south-west, and from
the south-west, as the wind shifts round northward to north-east, the
barometer rises—it rises to west, north-west, north, and north-east.
“The effect of the wind thus shifting round when traced upon paper by a
curve, seems certainly wave-like to the eye; but I believe it to be
simply consequent on the wind shifting round the compass, and
indicating alteration in the barometric column.
“If the wind remained north-east, say three weeks, there would be no
wave at all—there would be almost a straight line along a diagram
(varying only a little for _strength_). The atmospheric line, in such a
case, remains at the same height, and the barometer remains at 30
inches and (say) some three or four-tenths, for weeks together. So
likewise when the wind is south-westerly a long time, or near that
point, the atmospheric line remains _low_, towards 29 inches. Thus,
such ‘atmospheric waves’ may be an optical delusion.
“The diagram alluded to above shows how the barometer and thermometer
may be used in connection with each other in foretelling wind, and
consequently weather, that is coming on, because _as the one rises, the
other_ generally _falls_, and if you take the two together and confront
with their indications the amount of moisture in the air at any time,
you will scarcely be mistaken in knowing what kind of weather you are
likely to have for the _next two or three days_, which for the
gardener, the farmer, soldier, sailor, and traveller must be frequently
of considerable importance.”[14]
[Footnote 14: The late Admiral Fitzroy.]
We are indebted to M. Buys Ballot, a Dutch meteorologist, for an
invaluable generalization, the importance of which it is almost
impossible to over-estimate. This distinguished _savant_ says:—“It is
a fact above all doubt that the wind that comes is nearly at right
angles to the line between the places of highest and lowest barometer
readings. The wind has the place of lowest barometer at its left hand,
and is stronger in proportion as the difference of barometer readings
is greater.” These facts have been variously stated by other writers;
for example: “Stand with your back to the wind, and the barometer will
be lower on your left hand than on your right;” “Facing the wind the
centre of depression bears in the right-hand direction,” statements
which can be verified at any time by a brief study of the “Weather
Charts” now published in the daily journals. The value of the law
consists in its connecting the surface winds of our planet with the
actual pressure of the air itself, and it admits of the following
tabulation:—
+-------------+-------------+-------------+-------------+
| The wind is | The wind is | The wind is | The wind is |
| NORTHERLY | SOUTHERLY | EASTERLY | WESTERLY |
| when the | when the | when the | when the |
| BAROMETER | BAROMETER | BAROMETER | BAROMETER |
| is, in the | is, in the | is, in the | is, in the |
| N. ⎫ | N. ⎫ | N. High. | N. Low. |
| & ⎬ about | & ⎬ about | S. Low. | S. High. |
| S. ⎭ equal. | S. ⎭ equal. | E. ⎫ | E. ⎫ |
| E. Low. | E. High. | & ⎬ about | & ⎬ about |
| W. High. | W. Low. | W. ⎭ equal. | W. ⎭ equal. |
+-------------+-------------+-------------+-------------+
which can be verified by the reader from the daily Weather Charts in
the newspapers.
The above are deductions from Buys Ballot’s Law, still further
impressed on the memory by taking four outline maps of the British
Isles, inserting the names of Thurso, Penzance, Yarmouth, and Valentia,
with barometer readings of the kind above named at each place, and then
drawing a large arrow in red ink across the centre of each map in the
direction appropriate to the readings.
Mr. Strachan, in his able pamphlet on “Weather Forecasts,” puts the
matter thus: “It follows from Ballot’s Law that in the northern
temperate zone the winds will circulate around an area of low
atmospherical pressure in the _reverse direction_ to the movement of
the hands of a watch, and that the air will flow away from a region of
high pressure, and cause an apparent circulation of the winds around
it, _in the direction_ of watch hands.” And as the result of a careful
digest of data contained in the eleventh number of meteorological
papers, published by the Board of Trade, he has established the
following valuable propositions. As introductory to the propositions,
it should be stated that the positions of observations were the
following:—
Places. Latitude. Longitude.
Nairn 57° 29´ N. 4° 13´ W.
Brest 48 „ 28 4 „ 29 W.
Valentia 51 „ 56 10 „ 19 W.
Yarmouth 52 „ 37 1 „ 44 E.
Portrush (or Greencastle) 55 „ 12 6 „ 40 W.
Shields 55 „ 0 1 „ 27 W.