Robinson&rsquos 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 lace. 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.

“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	Â	687
readings of an Anemometer for 13 days	Â	773
will illustrate the method of making	Â	822
the entries, &c.	Â	855
“In this instance, the first read-	Â	900
ing (687) is less than the last (793).	Â	953
When the first reading is greater than	Â	990
the last, it will be necessary to borrow	Â	*066
1,000 in making the subtractions,	Â	197
and then deduct one from the number	Â	323
of stars. Thus, if the first reading	Â	414
of the series on the margin had	Â	597
been 887, the result would have been	Â	712
906 instead of 1106.	Â	793
Â		1106	  thousands of revolutions.
Â		2
Â	13	  2212	  miles of wind in period.
Â		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.

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 V² = 200 × P. From this formula, therefore, the pressure can be calculated corresponding to the observed velocity.