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 lbs. | Common designation 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.
Fig. 87.
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.
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.