THE INTERNAL WORK OF THE WIND. 



A second instrument was a very light anemometer, having paper cups, of 

 standard pattern and diameter, the weight of anus and cups being only 74 

 grammes, and its moment of inertia 8,604 gr. on.* 



With this instrument, a number of observations were taken, when it was lost 

 by being blown away in a gale. It was succeeded in its use by one of my own 

 construction, which was considerably lighter. This was also blown away. I after, 

 ward employed one of the same size as the standard pattern, weighing 48 grammes, 

 having a moment of inertia of 11,940 gr. cm. 8 , and finally I constructed one of one 

 half the diameter of the standard pattern, employing cones instead of hemispheres, 

 weighing 5 grammes, and having a moment of inertia of but 300 gr. cm. 2 



In the especially light instruments, the electric record was made at every half- 

 revolution, on an ordinary astronomical chronograph, placed upon the floor of the 

 Tower, connected with the anemometers by an electric circuit. Observations were 

 made on January 14, 1893, during a light wind having a velocity of from 9 to 

 17 miles an hour ; on January 25 and 26, during a moderate wind having a 

 velocity of from 16 to 28 miles an hour ; and on February 4 and 7, during a 

 moderate and high wind ranging from 14 to 36 miles an hour. Portions of these 

 observations are given on Plates II., III., and IV. A short portion of the record 

 obtained with the standard Weather Bureau anemometer during a high north- 

 west wind is given on Plate V. 



A prominent feature presented by these diagrams is that the higher the abso- 

 lute velocity of the wind, the greater the relative fluctuations which occur in it. 

 In a high wind the air moves in a tumultuous mass, the velocity being at one 

 moment perhaps 40 miles an hour, then diminishing to an almost instantaneous 

 calm, and then resuming.* 



The fact that an absolute local calm can momentarily occur during the prev- 

 alence of a high wind, was vividly impressed upon me during the observations of 

 February 4, when chancing to look up to the light anemometer, which was 

 revolving so rapidly that the cups were not separately distinguishable, I saw 

 them completely stop for an instant, and then resume their previous high speed of 

 rotation, the whole within the fraction of a second. This confirmed the suspicion 

 that the chrouographic record, even of a specially light anemometer, but at most 

 imperfectly notes the sharpness of these internal changes. Since the measured 

 interval between two electric contacts is the datum for computing the velocity, an 

 instantaneous stoppage, such as I accidentally saw, will appear on the record 

 simply as a slowiug of the wind, and such very significant facts as that just noted, 

 will be necessarily slurred over, even by the most sensitive apparatus of this kind. 

 * An example of a very rapid change may be seen on Plate IV., at 12.23 P - M - 



