S. P. Langley — Internal Work of the Wind. 47 



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

 ordinary small Robinson's anemometer, having cups 3 inches 

 (7"5 cm ) in diameter, the centre of the cups being G£ inches 

 (16f cm ) 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 Observatory, 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 irregularities 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 connec- 

 tion, and these chronograph sheets were measured and the 

 results tabulated. A portion of the record obtained on July 

 16, 1887, is given on Plate I, the abscissae representing time, 

 and the ordinates wind velocities. The observed points repre- 

 sent 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 convenience 

 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, 

 owing to the weight and inertia of the anemometer, this is far 

 from doing justice to the actual irregularities of the wind ; on 

 the other, it equally shows that the wind was far from being a 

 body of even approximate uniformity of motion, and that 

 even when considered in quite small sections, the motion was 

 found to be irregular almost beyond conception, — certainly 

 beyond anticipation ; for this record is not selected to repre- 

 sent an extraordinary breeze, but the normal movement of an 

 ordinary one. 



By an application of these facts, to be presented later, I then 

 reached by these experiments, the conclusion that it was theo- 

 retically possible to cause a heavy body wholly immersed in 

 the wind to be driven in the opposite direction, e. g. to move 

 east while the wind was blowing west, without the use of any 

 power other than that which the wind itself furnished, and 

 this even by the use of plane surfaces, and without taking 

 advantage of the more advantageous properties of curved ones. 



