Part 17

“The velocity is an approximation as near as can be obtained, from the values assigned by Neumayer, Stow, Laughton, Scott, Harris, James, &c.”

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.

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.

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.

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.

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 14r, 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; 2rrllt, 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.

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.