geographical miles apart. Valentia and Yarmouth are nearly on the same
parallel of latitude, about 450 miles apart. Portrush and Shields,
distant 180 miles, are on a parallel which is nearly as remote from the
parallel of Nairn as that of Valentia and Yarmouth is from the one
passing through Brest; and Shields is about as much to the westward of
Yarmouth as Portrush is to the eastward of Valentia. When observations
have not been obtainable for Brest, those made at Penzance have been
used instead.
_Proposition 1._—Whenever the atmospherical pressure is greater at
Brest than at Nairn, while it is of the same or nearly the same value
at Valentia and Yarmouth, being gradually less from south to north, the
winds over the British Isles are _westerly_.
_Proposition 2._—Whenever the pressure at Nairn is greater than at
Brest, while its values at Valentia and Yarmouth are equal, or nearly
so, the winds over the British Isles are _easterly_.
_Proposition 3._—Whenever the pressure at Valentia is greater than at
Yarmouth, while its values at Brest and Nairn are nearly equal, the
winds over the British Isles are _northerly_.
_Proposition 4._—Whenever the pressure at Yarmouth exceeds that at
Valentia, while there is equality of pressure at Nairn and Brest, the
winds of the British Isles are _southerly_.
_Proposition 5._—Whenever the pressure of the atmosphere is equal, or
nearly so, at Brest, Valentia, Nairn, and Yarmouth, and generally
uniform, the winds over the British Isles are variable in direction and
light in force.
The data from which the foregoing propositions were deduced, and indeed
all other cases calculated by Mr. Strachan, show in every well marked
instance that when the atmospherical pressure was
(1) greater in the south than in the north, the wind had westing;
(2) greater in the north than in the south, the wind had easting;
(3) greater in the east than in the west, the wind had southing;
(4) greater in the west than in the east, the wind had northing;
(5) uniformly high, or uniformly low, variable light winds (with
fine weather in the former case, and vapoury or wet weather in
the latter).
Conditions (1) and (3) give winds from the S.W. quarter.
Conditions (1) and (4) give winds from the N.W. quarter.
Conditions (2) and (4) give winds from the N.E. quarter.
Conditions (2) and (3) give winds from the S.E. quarter.
These principles may be employed to set forth the mode of foretelling
the impending change of wind as regards its direction and force; for
the atmospherical pressure may change—
(_a_) uniformly over the whole area of observation;
(_b_) by increasing in the south, or (which causes a similar
statical force) by decreasing in the north;
(_c_) by increasing in the north, or (which has the same effect) by
decreasing in the south;
(_d_) by increasing in the west, or (which has the same effect) by
decreasing in the east;
(_e_) by increasing in the east, or (which has the same effect) by
decreasing in the west;
------------------------------------------------------------------------
Scale, 0 to 6.
|
|Pressure in pounds
| per square foot.
| |
| |Miles per hour.
| | |
| | |Seaman’s Nomenclature.
| | | |
| | | |Scale, 0 to 12.
| | | | |
| | | | | Beaufort Scale.
---+-----+---+---------+----+-------------------------------------------
0·0| 0·00| 2 |Calm | 0 |
| | | | |
0·5| 0·25| 5 |Light Air| 1 | Just sufficient to make steerage way.
| | | | |
| | | Breeze | |
1·0| 1·00|10 | Light | 2 | ⎧With which a ship with ⎫ 1 to 2 knots.
1·5| 2·25|15 | Gentle | 3 | ⎨ all sail set would go ⎬ 3 to 4 „
2·0| 4·00|20 | Moderate| 4 | ⎩ in smooth water. ⎭ 5 to 6 „
2·5| 6·25|27 | Fresh | 5 | ⎧ ⎫ Royals, &c.
3·0| 9·00|35 | Strong | 6 | ⎪In which ⎮ Single Reefs and T.G. Sails.
-- | -- |42 | -- -- | | ⎪ ⎮ Double Reefs and Jib, &c.
| | | | | ⎪ ⎮
| | | Gale | | ⎨ she could ⎬
3·5| -- |50 |Moderate | 7 | ⎪ ⎮
4·0|16·00|60 |Fresh | 8 | ⎪ ⎮ Triple Reefs, &c.
4·5|20·25|-- |Strong | 9 | ⎩ just carry⎭ Close Reefs and Courses.
5·0|25·00|70 |Whole | 10 | ⎧In which she could just bear close-reefed
| | | | | ⎩ Maintopsail and reefed Foresail.
5·5|30·25|80 |Storm | 11 | Under Storm Staysails or Trysails.
6·0|36·00|90 |Hurricane| 12 | Bare Poles.
---+-----+---+---------+----+-------------------------------------------
With (_a_) similar wind and weather will continue.
„ (_b_) winds will veer towards west.
„ (_c_) „ „ east.
„ (_d_) „ „ north.
„ (_e_) „ „ south.
“The probable strength of wind will be in proportion to the rate of
increase of statical force, or differences of barometrical readings.
The position of least pressure must be carefully considered; as, in
accordance with the law, the wind will blow around that locality. The
same remark applies to areas of high pressure, which, however, very
rarely occur in a well-defined manner over the British Isles.”
Referring to the table on page 76, the scale 0 to 6 was formerly used
by meteorological observers at land stations, and it was intended to
express, when the square of the grade was obtained, the pressure of the
wind as given in the second column.
“The velocity is an approximation as near as can be obtained, from the
values assigned by Neumayer, Stow, Laughton, Scott, Harris, James,
&c.”[15]
[Footnote 15: Strachan’s “Portable Meteorological Register,”
4th edition.]
Few meteorological axioms are better established than that which
embodies the fact that “every wind brings its weather,” and the primary
cause of wind being the motion of the air induced by rarefaction, it is
obvious that there is a constant tendency for the equatorial and polar
currents in any locality to establish an equilibrium, and this
consideration is found to facilitate weather predictions for extended
periods. Thus, in consequence of the unusual prevalence of _east_ winds
in the spring of 1862, a wet summer was predicted. The prediction was
fully borne out by an incessant continuance of _south-west winds_, with
clouded skies and the usual accompaniment of deluges of rain. These
winds continuing, with slight intermissions only, till the spring of
the following year, less than the usual number of south-west winds was
looked for during the summer; the result fully justified the
anticipation, the summer of 1863 being fine and warm, especially during
the earlier portion. Similarly, without committing the inaccuracies of
Murphy in 1838, the summer of 1877 may be reasonably expected to be a
dry and cool one from the long continuance of warm and wet months in
the winter of 1876-7.
The scientific research and mechanical ingenuity directed of late years
to producing trustworthy estimates of the direction, pressure, and
velocity of the wind, have resulted in the production of a series of
instruments, possessing great precision and accuracy.
[Illustration: 54.
Wind Vane. Scale
about 1/20.]
The _direction_ of the wind is indicated by vanes, a very efficient
form of which is shown at Fig. 54, the _velocity_ by revolving cups,
and the _pressure_ by the pressure plate and by calculation from the
known velocity.
The Pendulum Anemometer (Fig. 56) shows in a simple manner the
direction and pressure of the wind. The peculiarly shaped vane ensures
the surface of the swinging pressure plate B being always kept towards
the wind. The pendulum plate hangs, during a calm, quite vertically,
indicating zero, and as the pressure increases it will be raised
through all degrees of elevation from 1 to 12. The vane is perforated
with holes large enough to be visible at some distance from the ground,
the 5 and 10 being specially larger, so that the angle to which the
pressure plate is raised can be quickly noted.
[Illustration: 55.
Compass Bearings.
Scale about 1/20.]
There is a simple contrivance (for the convenience of travellers)
called a Portable Wind Vane, or Anemometer, It is furnished with a
compass and bar needle, &c., and will tell the true direction of the
wind to within a half point.
[Illustration: 56.
Prestel’s Pendulum Anemometer.
Scale about 1/12.]
[Illustration: 57.
Lind’s Anemometer. Scale about 1/5.]
Lind’s Anemometer or Wind Gauge ranks among the earliest forms of
instruments designed to estimate the force of the wind. It consists of
a glass syphon, the limbs of which are parallel to each other, mounted
on a vertical rod, on which it freely oscillates by the action of the
vane which surmounts it. The upper end of one limb of the syphon is
bent outward at right angles to the main direction, and the action of
the vane keeps this open end of the tube always towards the quarter
from whence the wind blows. Between the limbs of the syphon is placed a
scale graduated from 0 to 3 in inches and 10ths, the zero being in the
centre of the scale. When the instrument is used, it is only necessary
to fill the tube with water to the zero of the scale, and then expose
it to the wind. The natural consequence of wind acting on the surface
of the water is to depress it in one limb and raise it in the other,
and the sum of the depression and elevation is the height of a column
of water which the wind is capable of sustaining at the time of
observation. Sudden gusts of wind are apt to produce a jumping effect
on the water in the tube, and to diminish this the bend of the syphon
is contracted. A brass plate is attached to the foot of the instrument,
bearing the letters indicating the cardinal points of the compass, to
show the direction of the wind.
Dr. Robinson, of Armagh, introduced an instrument, in 1850, which
consists of four hemispherical copper cups attached to the arms of a
metal cross. The vertical axis upon which these are secured has at its
lower extremity an endless screw placed in gear with a train of wheels
and pinions. Each wheel is graduated respectively to 1/10th, 1 mile, 10
miles, 100 miles, 1,000 miles, and these revolve behind a fixed index,
the readings of which are taken according to the indications on the
dials.
Dr. Robinson entertained the theory that the cups (measuring from their
centres) revolved with one-third of the wind’s velocity; and this
theory having been fully supported by experiment, due allowance has
been made in graduating the wheels so that the true velocity is
obtained by direct observation.
In an improved form of this anemometer the hemispherical cups are
retained, but the index portion of the instrument consists of two
graduated concentric circles, the inner one representing five miles
divided into 10ths, and the outer one bearing 100 divisions, each of
which is equivalent to five miles. At the top of the dial is a fixed
index, which, as the toothed wheel revolves, marks on the inner circle
the miles (up to five) and 10ths of miles the wind has travelled, while
a movable index, which revolves with the wheel, indicates on the outer
circle the passage of every five miles.
[Illustration: 58.
Improved Anemometer. Scale about 1/5.]
This instrument can be made very portable by removing the arms bearing
the cups, when the whole may be packed with iron shaft in a case 15 ×
13 × 4 inches. It may be placed in any desired position by screwing the
iron shaft supplied with it into the hole provided for the purpose, and
fixing the apparatus on a pole or on an elevated stand, if possible, in
an open space exposed to the _direct_ action of the wind.
If, when placing the instrument, the hands stand at 0, the next reading
will, of course, show the number of miles the wind has traversed; but,
should they stand otherwise, the reading may be noted and deducted from
the second reading, thus: Suppose the fixed index points to 2·5 and the
movable index to 125, the reading after 12 hours may be 200 on the
outer circle and 3·0 on the inner circle: these added together yield
203. By deducting the previous reading 127·5, we have the true
reading—viz., 75·5 miles as the distance travelled by the wind.
Having obtained the velocity of the wind in this manner in miles per
hour, the table on page 83, from Col. Sir Henry James’s “Instructions
for Taking Meteorological Observations,” will enable the observer to
calculate the pressure in pounds per square foot.
WEATHER NOTATION.
The following letters are used to denote the state of the weather:—
_b_ denotes blue sky, whether with clear or slightly hazy atmosphere.
_c_ „ cloudy, that is detached opening clouds.
_d_ „ drizzling rain.
_f_ „ fog.
_h_ „ hail.
_l_ „ lightning.
_m_ „ misty, or hazy so as to interrupt the view.
_o_ „ overcast, gloomy, dull.
_p_ „ passing showers.
_q_ „ squally.
_r_ „ rain.
_s_ „ snow.
_t_ „ thunder.
_u_ „ ugly, threatening appearance of sky.
_v_ „ unusual visibility of distant objects.
_w_ „ wet, that is dew.
A letter repeated denotes much, as _rr_, heavy rain; _ff_, dense fog;
and a figure attached denotes duration in hours, as 14_r_, 14 hours’
rain.
By the combination of these letters all the ordinary phenomena of the
weather may be recorded with certainty and brevity.
_Examples._—_bc_, blue sky with less proportion of cloud; _cb_, more
cloudy than clear; 2_rrllt_, heavy rain for two hours, with much
lightning, and some thunder.
VELOCITY AND PRESSURE OF THE WIND.
The Pressure varies as the Square of the Velocity, or _P_ ∝
_V_^2. The Square of the Velocity in Miles per Hour multiplied
by ·500 gives the Pressure in lbs. per square Foot, or _V_^2 ×
·005 = _P_. The Square Root of 200 times the Pressure equals
the Velocity, or √(200 × _P_) = _V_.
The subjoined Table is calculated from this data, by COL. SIR HENRY
JAMES, of the Ordnance Survey Office.
+-------------------------------------------------------------------+
|Pressure in |
|lbs. per |
|Square Foot. |
| |Velocity in |
| |Miles |
| |per Hour. |
| | |Pressure in |
| | |lbs. per |
| | |Square Foot. |
| | | |Velocity in |
| | | |Miles |
| | | |per Hour. |
| | | | |Pressure in |
| | | | |lbs. per |
| | | | |Square Foot. |
| | | | | |Velocity in |
| | | | | |Miles |
| | | | | |per Hour. |
| | | | | | |Pressure in |
| | | | | | |lbs. per |
| | | | | | |Square Foot. |
| | | | | | | |Velocity in |
| | | | | | | |Miles |
| | | | | | | |per Hour. |
| | | | | | | | |Pressure in |
| | | | | | | | |lbs. per |
| | | | | | | | |Square Foot. |
| | | | | | | | | |Velocity |
| | | | | | | | | |in Miles |
| | | | | | | | | |per Hour.|
+-----+------+-----+------+-----+------+-----+------+-----+---------+
| oz. | | lbs.| | lbs.| | lbs.| | lbs.| |
| 0·08| 1·000| 6·75|36·742|17·75|59·581|28·75|75·828|39·75| 89·162 |
| 0·25| 1·767| 7·00|37·416|18·00|60·000|29·00|76·157|40·00| 89·442 |
| 0·50| 2·500| 7·25|38·078|18·25|60·415|29·25|76·485|40·25| 89·721 |
| 0·75| 3·061| 7·50|38·729|18·50|60·827|29·50|76·811|40·50| 90·000 |
| 1·00| 3·535| 7·75|39·370|18·75|61·237|29·75|77·136|40·75| 90·277 |
| 2·00| 5·000| 8·00|40·000|19·00|61·644|30·00|77·459|41·00| 90·553 |
| 3·00| 6·123| 8·25|40·620|19·25|62·048|30·25|77·781|41·25| 90·829 |
| 4·00| 7·071| 8·50|41·231|19·50|62·449|30·50|78·102|41·50| 91·104 |
| 5·00| 7·905| 8·75|41·833|19·75|62·819|30·75|78·421|41·75| 91·378 |
| 6·00| 8·660| 9·00|42·426|20·00|63·245|31·00|78·740|42·00| 91·651 |
| 7·00| 9·354| 9·25|43·011|20·25|63·639|31·25|79·056|42·25| 91·923 |
| 8·00|10·000| 9·50|43·588|20·50|64·031|31·50|79·372|42·50| 92·195 |
| 9·00|10·606| 9·75|44·158|20·75|64·420|31·75|79·686|42·75| 92·466 |
|10·00|11·180|10·00|44·721|21·00|64·807|32·00|80·000|43·00| 92·736 |
|11·00|11·726|10·25|45·276|21·25|65·192|32·25|80·311|43·25| 93·005 |
|12·00|12·247|10·50|45·825|21·50|65·574|32·50|80·622|43·50| 93·273 |
|13·00|12·747|10·75|46·368|21·75|65·954|32·75|80·932|43·75| 93·541 |
|14·00|13·228|11·00|46·904|22·00|66·332|33·00|81·240|44·00| 93·808 |
|15·00|13·693|11·25|47·434|22·25|66·708|33·25|81·547|44·25| 94·074 |
| | |11·50|47·958|22·50|67·082|33·50|81·853|44·50| 94·339 |
| lbs.| |11·75|48·476|22·75|67·453|33·75|82·158|44·75| 94·604 |
| 1·00|14·142|12·00|48·989|23·00|67·823|34·00|82·462|45·00| 94·868 |
| 1·25|15·811|12·25|49·497|23·25|68·190|34·25|82·764|45·26| 95·393 |
| 1·50|17·320|12·50|50·000|23·50|68·556|34·50|83·066|45·50| 95·131 |
| 1·75|18·708|12·75|50·497|23·75|68·920|34·75|83·366|45·75| 95·655 |
| 2·00|20·000|13·00|50·990|24·00|69·282|35·00|83·666|46·00| 95·916 |
| 2·25|21·213|13·25|51·478|24·25|69·641|35·25|83·964|46·25| 96·176 |
| 2·50|22·360|13·50|51·961|24·50|70·000|35·50|84·261|46·50| 96·436 |
| 2·75|23·452|13·75|52·440|24·75|70·356|35·75|84·567|46·75| 96·695 |
| 3·00|24·494|14·00|52·915|25·00|70·710|36·00|84·852|47·00| 96·953 |
| 3·25|25·495|14·25|53·385|25·25|71·063|36·25|85 146|47·25| 97·211 |
| 3·50|26·457|14·50|53·851|25·50|71·414|36·50|85·440|47·50| 97·467 |
| 3·75|27·386|14·75|54·313|25·75|71·763|36·75|85·732|47·75| 97·724 |
| 4·00|28·284|15·00|54·772|26·00|72·111|37·00|86·023|48·00| 97·979 |
| 4·25|29·154|15·25|55·226|26·25|72·456|37·25|86·313|48·25| 98·234 |
| 4·50|30·000|15·50|55·677|26·50|72·801|37·50|86·602|48·50| 98·488 |
| 4·75|30·822|15·75|56·124|26·75|73 143|37·75|86·890|48·75| 98·742 |
| 5·00|31·622|16·00|56·568|27·00|73·484|38·00|87·177|49·00| 98·994 |
| 5·25|32·403|16·25|57·008|27·25|73·824|38·25|87·464|49·25| 99·247 |
| 5·50|33·166|16·50|57·415|27·50|74·161|38·50|87·749|49·50| 99·498 |
| 5·75|33·911|16·75|57·879|27·75|74·498|38·75|88·034|49·75| 99·749 |
| 6·00|34·641|17·00|58·309|28·00|74·833|39·00|88·317|50·00| 100·000 |
| 6·25|35·355|17·25|58·736|28·25|75·166|39·25|88·600| | |
| 6·50|36·055|17·50|59·160|28·50|75·498|39·50|88·881| | |
+-----+------+-----+------+-----+------+-----+------+-----+---------+
This is the only table hitherto much in use for converting velocity
into pressure, and was prepared by Smeaton and others. It does not,
however, express the true relation, which has yet to be determined.
The Anemograph, or Self-Recording Wind Gauge, has for its object the
registration of the velocity and direction of the wind from day to day.
Figs. 59 and 60 show the form designed and arranged by Mr. Beckley, of
the Kew Observatory, which has been adopted by the Meteorological
Office.
[Illustration: 59.
Anemograph. Scale about 1/20.
Portion for exterior of observatory.]
It consists of a set of hemispherical cups and vanes, which are exposed
on the roof of the house, and of the recording apparatus, which is
placed inside the house.
The motion imparted to the hemispherical cups by the wind is
communicated to the steel shaft B, which, passing through the hollow
shaft C, and having at its lower end an endless screw, works into a
series of wheels in the iron box D, which reduces the angular velocity
7,000 times. At the required distance the motion, having emerged at E,
is connected with F, where, by means of bevelled wheels, it moves the
spiral brass registering pencil C, which is arranged so that each
revolution records 50 miles of velocity on the prepared paper H.
The direction of the wind is indicated by the arrow L, which is kept in
position by the fans M. These communicate, by an endless screw and
train of wheels, through the shaft C and the box D to the recording
apparatus, consisting of a spiral brass pencil, which in one revolution
records variations through the cardinal points of the compass, on the
same prepared paper as that which receives the record of velocity.
[Illustration: 60.
Anemograph. Scale about 1/20.
Portion for interior of observatory.]
The paper is held on the drum by two small clips, and may be readily
changed, by unclamping the cross V, without disturbing the drum or any
other part of the instrument.
[Illustration: 61.
Self-recording Magnetometer, Kew Observatory.]
VI.—ELECTRIFICATION.
William Gilbert, a physician of Colchester, first showed in 1600 that
the earth as a whole has the properties of a magnet, and consequently
that the directive action exerted by it upon a compass needle
represents only a special case of the mutual action of two magnets. In
1845, Faraday established the fact that susceptibility to magnetic
force is not, as was generally believed, confined to iron, nickel, and
a few other substances, but is a property of all substances. According
to Balfour Stewart, auroræ and earth currents may be regarded as
secondary currents resulting from changes in the earth’s magnetism.
Magnetic phenomena are included under the general term terrestrial
magnetic elements, and consist of magnetic declination, inclination,
and intensity.
These are for convenience determined separately; the first by an
instrument called a _Declinometer_, and the second by an _Inclinometer_
or _Dipping Needle_. The Declinometer is also made to serve the
additional purpose of measuring the _intensity_ of the earth’s magnetic
force, which it effects on a principle similar to that by which the
force of gravity is determined by the oscillations of a pendulum of
known length on any given portion of the earth’s surface. The
declinometer needle is made to oscillate, and the number of
oscillations in a given time counted; due allowance being made for the
strength of the needle, it is obvious that the force which restores the
needle to rest can be estimated. To ascertain the angle of
_declination_, the zero line of the compass card is made to coincide
with the geographical north and south line; and the angle which the
direction of the needle makes with this line is then read off on a
graduated circle over which the needle turns. The magnetic
_inclination_ or _dip of the needle_ is estimated by observing the
inclination to a horizontal plane of a needle turning on the vertical
plane which passes through the magnetic north and south points.
[Illustration: 62.]
Fig. 62 shows a simple form of magnetic needle suspended on a fine
steel point, which is supported by a brass stand; the addition of a
graduated circle would constitute such an arrangement a Declinometer.
[Illustration: 63.]
[Illustration: 64.]
Fig. 63 gives the appearance of the dipping needle, or Inclinometer,
and Fig. 64 an arrangement by which both kinds of terrestrial as well
as local attraction may be shown.
These components of the earth’s magnetism undergo not only an annual
but a daily and even hourly variation, apparently connected in some
occult manner with the frequency of the sun’s spots. The needle
sometimes suffers such exceptional perturbations as to suggest the idea
of a magnetic storm. These disturbances are usually accompanied (in
polar regions) by luminous phenomena called auroræ. Continuous
automatic records of them, therefore, is of great value, as
facilitating inductive research which may lead to valuable practical
results.
Accordingly the Royal Society have adopted for the Kew and other
observatories the form of Magnetograph, or Self-recording Magnetometer,
shown at Fig. 61, by means of which the variations just referred to are
registered by the oscillations of three magnets on photographically
prepared paper, stretched on a drum revolved by clockwork.
One magnet is suspended in the magnetic meridian by a silk thread, and,
by the aid of a mirror attached, it describes on the cylinder, moved by
clockwork in the centre pier, all the variations in the magnetic
_declination_.
The other two components of the magnetic force of the earth are given
by the other magnets. That recording the vertical variations rests on
two agate edges under a glass shade, while the horizontal component
magnet is suspended by a double silk thread, under the shade to the
right of the picture, being retained by the tension of the thread in a
position nearly at right angles to the magnetic meridian.
The clock box in the centre covers the three revolving cylinders
bearing the sensitive photographic paper, and to each magnet is
attached a semicircular mirror, which reflects the rays from a gas jet
to one of the cylinders, and thus describes by a curved line the
oscillations of the magnet. A second semicircular mirror is _fixed_ to
the pier on which the instrument stands, and consequently describes a
straight line, or zero, from whence the curves are measured.
To avoid errors attending sudden changes of temperature, underground
vaults are always chosen for magnetic observations, and also on account
of light being more easily and perfectly excluded.
ATMOSPHERIC ELECTRICITY.