APPARATUS EMPLOYED FOR REGISTERING THE DIRECTION, PRESSURE, AND VELOCITY
OF THE WIND.
=122. The Vane.=--The instrument by which the wind's direction is most
generally noted, is the vane, or weather-cock, and all that need be said
of it here is that the points north, east, south and west, usually
attached to it, should indicate the _true_ and not the _magnetic_
directions; and that care should be taken to prevent its setting fast.
Very complicated instruments are required for ascertaining the pressure
and velocity of the wind, and these are called _Anemometers_. The simplest
is _Lind's_.
[Illustration: Fig. 86.]
=123. Lind's Anemometer, or Wind-Gauge= (fig. 86), invented so late as
1775, for showing the pressure of the wind, consists of a glass syphon,
the limbs parallel to each other, and each limb the same diameter. One end
of the syphon is bent at right angles to the limb, so as to present a
horizontal opening to the wind. A graduated scale, divided to inches and
tenths, is attached to the syphon tube, reading either way from a zero
point in the centre of the scale. The whole instrument is mounted on a
spindle, surmounted by a vane, and is moved freely in any direction by the
wind, always presenting the open end towards the quarter from which the
wind blows. To use the instrument, it is simply filled up to the zero
point with water, and then exposed to the wind; the difference in the
level of the water gives the force of the wind in inches and tenths, by
adding together the amount of depression in one limb, and elevation in the
other, the _sum of the two_ being the height of a column of water which
the wind is capable of sustaining at that time.
TABLE,
Showing the Force of Wind on a square foot, for different heights of the
column of Water in Lind's Wind-Gauge.
+-----------------------------------+
|Inches.|Force in|Common designation|
| | lbs. | of such Wind. |
|-------+--------+------------------|
| 6 | 31.75 | A Hurricane. |
| 5 | 26.04 | A violent Storm. |
| 4 | 20.83 | A great Storm. |
| 3 | 15.62 | A Storm. |
| 2 | 10.42 | A strong Wind. |
| 1 | 5.21 | A high Wind. |
| .5 | 2.60 | A brisk Wind. |
| .1 | .52 | A fresh Breeze. |
| .05 | .26 | A gentle Breeze. |
| 0. | 0. | A Calm. |
+-----------------------------------+
=124. Modification of Lind's Gauge.=--_Sir W. Snow Harris_ has effected a
modification of Lind's anemometer, with a view of obtaining a hand
instrument for use at sea more especially. At present the force of the
wind is estimated at sea by an arbitrary scale, suggested by Sir F.
Beaufort, the late hydrographer; 0 being calm, 12 the strongest hurricane,
and the intermediate numerals giving the varying strength of the wind.
There has been a long-felt want of instrumental means for obtaining this
data at sea, if merely for the sake of checking occasionally personal
estimations, which may vary considerably among different observers.
Harris's wind gauge is intended to be held by hand, while facing the wind,
and keeping it in proper position by attending to a spirit-level attached.
When in position, and held firmly, the tube has to be opened to the wind
by pressure of the thumb acting upon jointed levers, controlled by
springs. The pressure of the wind moves the enclosed liquid; and by
withdrawing the thumb, the tube is closed so as to keep the liquid in its
position; the reading is then taken from its scale, either in pounds on
the square foot, miles per hour, or the ordinary designations of wind, as
light, fresh, strong, &c.
=125. Robinson's Anemometer.=--_Dr. Robinson_, of Armagh, is the inventor
of a very successful anemometer, which determines the horizontal velocity
of the wind. It was first used in 1850, in the meteorological and tidal
observations made on the coast of Ireland under the direction of the Rev.
Dr. Lloyd. No meteorological observatory should be without this valuable
instrument, which is essential in determining the average velocity of the
wind of a locality as distinguished from the most frequent wind of the
same place. It is represented in fig. 87. Four hollow hemispherical cups,
_A A_, are extended upon conjugate diameters, or arms, with their
diametrical planes placed vertically, and facing the same way upon a
vertical axis, _B_, which has at its lower extremity an endless screw,
_D_. The axis is supported at _C_ so as to turn with as little friction as
possible. The endless screw is placed in gear with a train of wheels and
pinions. Each wheel carries an index over a stationary dial in front; or
the index is fixed, and the graduations are placed upon the wheels
themselves.
[Illustration: Fig. 87.]
Dr. Robinson has proved, both by theory and experiment, that the centre of
any one of the cups so mounted and set in motion by the wind, revolves
with one-third of the wind's velocity. If, therefore, the diametrical
distance between the centres of the cups be one foot, the circle described
by the centres in one revolution is 3.1416 feet, and the velocity of the
wind will be three times this, or 9.42 feet, which must be referred to
time for the absolute rate. The instrument is sometimes made with the
centres of the cups 1.12 feet apart, so that the circle described is
1/1500 of a mile in circumference. Hence, to produce one revolution of the
cups, the wind must travel three times as fast, or 1/500 of a mile.
Therefore, 500 revolutions will be produced by one mile of wind; so that
the dials may be graduated to register the velocity in miles and tenths of
miles. The simplest arrangement is with five dials, recording respectively
10, 100, 1,000, 10,000 and 100,000 revolutions.
_Directions for using Robinson's Anemometer._--The dials read off in the
same manner as the register of a gas meter, commencing with the dial
farthest from the endless screw.
"The figures on the first dial indicate so many hundreds of thousands of
revolutions; those on the second dial so many tens of thousands; those on
the third, thousands; those on the fourth, hundreds; and those on the
fifth so many tens.
"The instrument should be read every morning at 9 o'clock; and, usually,
it will only be necessary to read the first three dials. The figures can
be entered as they are read off. Should the index point _between_ two
figures, the less of the two is to be taken.
"For example, if the first dial points to 7, or between 7 and 8; while the
second dial indicates 4; and the third, 5; the entry to be made is 745
(indicative of 745 _thousand_ revolutions).
"Every time the index of the first dial is found to have passed zero (0),
a cross or star is to be prefixed to the next (a lower) reading.
"To ascertain how many _thousands_ of revolutions have been made during
the month, it will simply be necessary to subtract the first reading from
the last, and prefix to the three figures thus obtained a figure
corresponding to the number of stars in the column. For every _thousand_
revolutions there are two miles of wind: we have therefore only to
multiply by 2 to find how many miles of wind have passed during the month.
"Two entries must be made for the last day of each month (the one being
written under the other), so as to bring the readings down to 9 A.M. on
the 1st of the following month. The same entry which ends one month, will
therefore begin the next. This repetition of one entry is necessary, in
order to prevent losing a day's wind.
"The accompanying example of the 687
readings of an Anemometer for 13 days 773
will illustrate the method of making 822
the entries, &c. 855
900
"In this instance, the first reading 953
(687) is less than the last (793). 990
When the first reading is greater than *066
the last, it will be necessary to borrow 197
1,000 in making the subtractions, 323
and then deduct one from the number 414
of stars. Thus, if the first reading 597
of the series on the margin had 712
been 887, the result would have been 793
906 instead of 1106. ----
1106 thousands of revolutions.
2
+-----
13 | 2212 miles of wind in period.
+-----
170 miles of wind per day, on
an average.
"The foregoing directions are all which require to be regularly attended
to. But it may be interesting at times to find the velocity of the wind
during a period of a few minutes. This may be ascertained by observing the
difference of two readings of all the dials, with an interval of some
minutes between them, when a very brief calculation will suffice; but
perhaps the simplest method is the following:--
"Take two readings, with an interval of 12 minutes between them. The
difference of these readings, divided by 10, is the velocity of the wind
in miles per hour. Thus--if the reading of the five dials (from left to
right) at noon is 15206, and at 12 minutes past 12 is 15348, the velocity
of the wind is 14.2 miles per hour."--_Admiral FitzRoy, F.R.S._
A lever and clutch are sometimes fitted to this anemometer, as in fig. 88,
for throwing the train out of gear when not required to register. It may
also be connected with clock-work so as to be self-recording, by causing
the mechanism to impress a mark upon prepared paper moved by the
apparatus, at certain intervals of time.
[Illustration: Fig. 88.]
This anemometer should be fixed in an exposed situation, as high above
ground as may be convenient for reading. It may be made very portable, by
the arms which carry the cups being fitted to unscrew or to fold down.
When fitted in gimbals, it can be used at sea with much advantage.
The pressure of the wind has been experimentally proved to vary as the
square of the velocity; the relation being _V squared_ = 200 x _P_. From this
formula, therefore, the pressure can be calculated corresponding to the
observed velocity.
=126. Whewell's Anemometer.=--This apparatus, the invention of the
celebrated Dr. W. Whewell, registers the horizontal motion of the air with
the direction. Its mechanism may be described in general terms, as
follows:--
A horizontal brass plate is attached to a vertical spindle, which passes
through the axis of a fixed cylinder, being supported by a bearing at the
lower end, and working in a collar at the upper. A vane is attached, by
which the plate is moved about according to the direction of the wind. A
fly, having eight fans, each fixed at an angle of 45 deg. with the axle, is
placed upon the plate so that the axle is in the line of direction of the
vane. An endless screw on the axle turns a vertical wheel having one
hundred teeth, the axle to which has also an endless screw working into a
horizontal wheel, having a like number of teeth, and which communicates
motion to a vertical screw fifteen inches long. On this screw is placed a
moveable nut, which carries a pencil. Round the cylinder is wrapped daily
a paper divided for the points of the compass. The wind acting upon the
vane will cause the plate to turn; and the screw which carries the pencil
will travel with it, so that the pencil will mark upon the paper the
direction of the wind. The fly will also be set in motion, and thereby the
nut upon the screw will descend, so that the attached pencil will trace a
vertical line upon the paper. When the fans on the axle are 2.3 inches
from axis to end, and 1.9 inches wide, and the thread of the screw such
that forty-five revolutions will cause the nut to descend two inches,
75.85 miles of wind will cause the pencil to descend through a vertical
space of two inches; but the actual trace upon the paper will be longer in
proportion to the magnitude of change of azimuth, or direction, of the
wind.
=127. Osler's Anemometer, and Pluviometer.=--Mr. Follet Osler is the
inventor of a self-recording apparatus which registers the direction and
pressure of the wind, and the amount and duration of rain, upon the same
sheet of paper. His apparatus has met with very much approbation, and has
been erected in many observatories. The mechanism may be modified in
various ways, and the following is a description of the simplest and most
recent arrangement.
[Illustration: Fig. 89.]
The instrument, of which fig. 89 is a diagram rather than a picture,
consists, first, of a vane, _V_, of a wedge-shape form, which is found to
answer better than a flat vane; for the latter is always in a neutral
line, and therefore is not sufficiently sensitive. A wind-mill governor
has been substituted for the vane to get the direction of the wind, with
advantage. At the lower end of the tube, _T T_, is a small pinion, working
in a rack, _r_, which moves backwards and forwards as the wind presses the
vane. To this rack a pencil, _x_, is attached, which marks the direction
of the wind on a properly ruled paper, placed horizontally beneath, and so
adjusted as to progress at the rate of half an inch per hour, by means of
a simple contrivance connecting it with a good clock. The paper is shown
in the illustration upon the table of the instrument.
The pressure plate, _F_, for ascertaining the force of the wind, is one
foot square, placed immediately beneath, and at right angles with the
vane; it is supported by light bars, running horizontally on friction
rollers, and communicating with flattened springs, 1, 2, 3, so that the
plate, when affected by the pressure of the wind, acts upon them, and they
transfer such action to a copper chain passing down the interior of the
direction tube, and over a pulley at the bottom. A light copper wire
connects this chain with the spring lever, _y y_, carrying a pencil which
records the pressure upon the paper below. Mr. Osler much prefers a spring
to any other means for ascertaining the force of the wind, because it is
of the highest importance to have as little matter in motion as possible,
otherwise the momentum acquired will cause the pressure plate to give very
erroneous indications. The pressure plate is as light as is consistent
with strength. It is kept before the wind by the vane, and is urged out by
three or more springs, so that with light winds one only is compressed,
and two, or more, according to the strength of the wind.
The _pluviometer_ is placed on the right in the figure, _P P_ being the
plane of the roof of the building. The rain funnel, _R_, exposes an area
of about 200 square inches. The water collected in it is conveyed by a
tube through the roof of the building into a glass vessel, _G_, so
adjusted and graduated as to indicate a quarter of an inch of rain for
every 200 square inches of surface, _i. e._ 50 cubic inches. _G_ is
supported by spiral springs, _b b_, which are compressed by the
accumulating rain. A glass tube, open at both ends, is cemented into the
bottom of _G_, and over it is placed a larger one closed at the top like a
bell glass. The smaller tube thus forms the long leg of a syphon, and the
larger tube acts as the short leg. The water, having risen to the level of
the top of the inner tube, drops over into a little copper tilt, _t_, in
the globe, _S_, beneath the reservoir. This tilt is divided into two equal
partitions by a slip of copper, and placed upon an axis not exactly
balanced, but so that one end or the other preponderates. The water then
drops into the end of the tilt which happens to be uppermost, and when
quite full it falls over, throwing the water into the globe, _S_, from
which it flows away by the waste pipe. In this way an imperfect vacuum is
produced in the globe, quite sufficient to produce a draught in the small
tube of the syphon, or the long leg; and the whole contents of the
reservoir, _G_, immediately run off, and the spiral springs, _b b_,
elevate the reservoir to its original position. To produce this action, a
quarter of an inch of rain must have fallen. The registration is easily
understood. A spring lever, _z_, carrying a pencil, is attached by a
cord, _c_, to _S_. This spring always keeps the cord tight, so that as the
apparatus descends during the fall of rain, the spring advances the pencil
more and more from the zero of the scale upon the paper beneath, until a
quarter of an inch has fallen, when the pencil is drawn back to zero by
the ascent of the reservoir.
The clock movement carries the registering paper forward by one of the
wheels working into a rack attached to the frame.
The adjustment of the instrument should be carefully made at its first
erection. The scale for pressure should be established experimentally, by
applying weights of 2, 4, 6, &c., lbs., to move the pressure plate.
The registration trace for twenty-four hours is readily understood. The
direction is recorded on the centre part; the pressure on one side, and
the rain on the other. Lines parallel to the length of the paper show no
rain, steady wind, and constant pressure. On the rain trace, a line
parallel to the width of the paper shows that the pencil had been drawn
back to zero, a quarter of an inch of rain having fallen. The hour lines
are in the direction of the width of the paper.
At the International Exhibition 1862, Messrs. Negretti and Zambra
exhibited an improved Osler's anemometer, having combined with it
Robinson's cups, so that the pressure and velocity appear on the same
sheet, on which a line an inch in length is recorded at every ten miles;
thus the complete instrument shows continuously the direction, pressure,
and velocity of the wind.
=128. Beckley's Anemometer.=--Mr. R. Beckley, of the Kew Observatory, has
devised a self-registering anemometer, which consists of three principal
parts: Robinson's cups for the determination of velocity; a double fan, or
wind-mill governor, for obtaining the direction; and a clock to move a
cylinder, around which registration paper is wrapped. The paper records
the time, velocity, and direction of the wind for twenty-four hours, when
it must be replaced. It has a cast-iron tubular support, or pedestal to
carry the external parts--the cups and the fans,--which must be erected
upon the roof of the building upon which it is desired to mount the
instrument.
The fans keep their axis at right angles to the wind; and with any change
of direction they move, carrying with them an outer brass tube, which
rests upon friction balls on the top of the pedestal, and is attached to a
tubular shaft passing through the interior of the pedestal, and
terminating with a mitre wheel. The mitre wheel, working with other cogged
wheels, communicates the motion of the direction shaft to a cylinder
carrying a pencil, to record the direction.
The shaft carrying the cups is supported upon friction balls, placed in a
groove formed on the top of the direction shaft, and passing through the
interior of that shaft, comes out below the mitre wheel, where it is
terminated in an endless screw, or worm.
Upon the wind moving the cups, motion is given to the innermost shaft,
thence to the worm-wheel, whence motion is given to a pencil which
registers the velocity.
De la Rue's metallic paper is used in registration, it having the property
of receiving a trace from a brass pencil. The pencils can, therefore, be
made in the most convenient form. Mr. Beckley forms each pencil of a strip
of brass wrapped round a cylinder, making a very thin threaded screw, so
that the contact of the pencil cylinder and the clock cylinder is a mere
point of the metallic thread. The pencil cylinders are placed side by side
upon the cylinder turned by the clock, and require no spring or other
appliance to keep them to their work, but always make contact with the
registration paper by their own gravity. They therefore require no
attention, and being as long as the trace which they make, they will last
a long time.
The velocity pencil has only one turn on the cylinder, and its pitch is
equal to a scale of fifty miles upon the paper. The direction pencil has
likewise one turn on its cylinder, its pitch being equal to a scale of the
cardinal points of the compass upon the paper.
The clock gives a uniform motion of half an inch per hour to the cylinder
upon which the paper is fastened.
The registering mechanism of the instrument is very compact, requiring
only a space of about 18 inches by 8 inches.
In the Report of the British Association for 1858, Mr. Beckley has given a
detailed description of his anemometer, with drawings of all the parts.
=129. Self-Registering Lind's Anemometer.=--A Lind's wind-gauge, designed
to register the maximum pressure, was exhibited at the International
Exhibition 1862, by Mr. E. G. Wood. The bend of the syphon is contracted
to obtain steadiness. On the leeward limb a hole is drilled corresponding
in size with the contracted portion of the tube. The edge of the hole
corresponds with the zero of the scale. On the pressure of the wind
increasing, as much of the water as would have risen above the aperture
flows away, and therefore the quantity left indicates the greatest
pressure of the wind since the last setting of the instrument, which is
done by filling it with water up to the zero point.
=130. Anemometric Observations.=--To illustrate the value of anemometric
observations, we quote from a paper by Mr. Hartnup, on the results
obtained from Osler's Anemometer, at the Liverpool Observatory. The six
years' observations, ending 1857, gave for the yearly average of the
winds: North-easterly, on 60 days, at 7.8 miles per hour; North-westerly,
on 112 days, at 15.4 miles per hour; South-easterly, on 115 days, at 11.0
miles per hour; South-westerly, on 77 days, at 13.8 miles per hour; and
one day calm. From the same observations, the average variation in the
strength of the wind during the 24 hours is:--11 miles per hour, the
minimum force, occurring at 1-1/2 a.m.; until 6 a.m. it remains much the
same, being then 11.3 miles per hour; at 10 a.m. it is 13.4 miles per
hour; at 1-1/2 p.m. the wind is at its maximum strength, being 14.8 miles
per hour; at 5 p.m. it is again 13.4 miles per hour, and at 9 p.m. 11.3
miles per hour. Hence it appears that the wind falls to its minimum force
much more gradually than it rises to its maximum; that the decrease and
increase are equal and contrary, so that the curve is symmetrical; and
that generally the force of wind is less at night than during the day.
"There is evidence," says Admiral FitzRoy, "in Mr. Hartnup's very valuable
anemometrical results, which seems to prove that to his observatory, in a
valley, with buildings and hills to the north-eastward, the real polar
current does not blow from N.E., but nearer S.E. By his reliable digest of
winds experienced there, it appears that those most prevalent were from
W.N.W. and S.S.E. But in England, generally, the _prevailing_ winds are
_believed_ to be westerly, inclining to south-westerly, and
north-easterly; while of all winds, the south-easterly is about the
rarest.
"At Lord Wrottesley's observatory, in Staffordshire, about 530 feet above
the sea, there appears to be considerably less strength of wind at any
given time, when a gale is blowing _generally_, than occurs simultaneously
at places along the sea-coast: whence the inference is, that undulations
of the land's surface and hills, diminish the strength of wind materially
by frictional resistance.
"All the synoptic charts hitherto advanced at the Board of Trade exhibit a
marked diminution of force inland compared with that on the sea-coast.
Indeed, the coast itself offers similar evidence, in its stunted, sloping
trees, and comparative barrenness."[14]