A Treatise on Meteorological Instruments Part 16

CHAPTER XIII.

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-c.o.c.k, 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_.

[Ill.u.s.tration: 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 gla.s.s 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 3175 A Hurricane. 5 2604 A violent Storm. 4 2083 A great Storm. 3 1562 A Storm. 2 1042 A strong Wind. 1 521 A high Wind. 5 260 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.

[Ill.u.s.tration: 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 31416 feet, and the velocity of the wind will be three times this, or 942 feet, which must be referred to time for the absolute rate. The instrument is sometimes made with the centres of the cups 112 feet apart, so that the circle described is 1/1500 of a mile in circ.u.mference. 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 pa.s.sed 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 pa.s.sed 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 repet.i.tion 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 ill.u.s.trate 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 142 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.

[Ill.u.s.tration: 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_ = 200 _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 bra.s.s plate is attached to a vertical spindle, which pa.s.ses 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 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 compa.s.s. 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 23 inches from axis to end, and 19 inches wide, and the thread of the screw such that forty-five revolutions will cause the nut to descend two inches, 7585 miles of wind will cause the pencil to descend through a vertical s.p.a.ce 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.

[Ill.u.s.tration: 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 subst.i.tuted 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 ill.u.s.tration 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 pa.s.sing 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 gla.s.s 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 acc.u.mulating rain. A gla.s.s 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 gla.s.s. 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 part.i.tions 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 princ.i.p.al 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 bra.s.s tube, which rests upon friction b.a.l.l.s on the top of the pedestal, and is attached to a tubular shaft pa.s.sing 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 b.a.l.l.s, placed in a groove formed on the top of the direction shaft, and pa.s.sing 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 bra.s.s pencil. The pencils can, therefore, be made in the most convenient form. Mr. Beckley forms each pencil of a strip of bra.s.s 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.