Internal Work of the Wind. 431 



Part II. — Experiments with the use of special apparatus. 



In the ordinary uses of the anemometer (let ns suppose 

 it to be a Robinson's anemometer, for illustration) the re- 

 gistry is seldom taken as often as once a minute ; thus, in 

 the ordinary practice of the United States Weather Bureau, 

 the registration is made at the completion of the passage of 

 each mile of wind. If there be very rapid fluctuations of the 

 wind, it is obviously desirable, in order to detect them, to 

 observe the instrument at very brief intervals, e.g. at least 

 every second, instead of every minute or every hour, and it is 

 equally obvious that in order to take up and indicate the 

 changes which occur in these brief intervals, the instrument 

 should have as little inertia as possible, its momentum tending 

 to falsify the facts, by rendering the record more uniform than 

 would otherwise be the case. 



In 1887 I made use of the only apparatus at command — 

 an ordinary small Robinson's anemometer, having cups 3 

 inches (7*5 centim.) in diameter, the centres of the cups being 

 6 j inches (16| centim.) from the centre of rotation. This 

 was placed at the top of a mast 53 feet (16*2 metres) in height, 

 which was planted in the grounds of the Allegheny Obser- 

 vatory, on the flat summit of a hill which rises nearly 400 feet 

 (122 metres) above the valley of the Ohio River. It was, 

 accordingly, in a situation exceptionally free from those ir- 

 regularities of the wind which are introduced by the presence 

 of trees and of houses, or of inequalities of surface. 



Every twenty-fifth revolution of the cups was registered by 

 closing an electric circuit, and the registry was made on the 

 chronograph of the Observatory by a suitable electric con- 

 nexion, and these chronograph sheets were measured and the 

 results tabulated. A portion of the record obtained on July 

 16, 1887, is given on Plate V., the abscissas representing time, 

 and the ordinates wind velocities. The observed points re- 

 present the wind's velocities as computed from the intervals 

 between each successive electrical contact, as measured on 

 the chronograph sheets, and for convenience in following the 

 succession of observed points they are here joined by straight 

 lines, though it is hardly necessary to remark that the change 

 in velocity is in fact, though quite sharp, yet not in general 

 discontinuous, and the straight lines here used for con- 

 venience do not imply that the rate of change of velocity is 

 uniform. 



The wind velocities during this period of observation ranged 

 from about 10 to 25 miles an hour, and the frequency of 

 measurement was every 7 to 17 seconds. If, on the one hand, 



