S. P. Langley — Internal Work of the Wind. 49 



were made on Jan. 14, 1893, during a light wind having a 

 velocity of from 9 to IT 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 mode- 

 rate 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 stan- 

 dard Weather Bureau anemometer during a high northwest 

 wind is given on Plate V. 



A prominent feature presented by these diagrams is that the 

 higher the absolute velocity of the wind, the greater the rela- 

 tive 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 prevalence of a high wind, was vividly impressed 

 upon me during the observations of February 4, when chanc- 

 ing 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 

 chronographic 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 instan- 

 taneous stoppage, such as I accidentally saw, will appear on 

 the record simply as a slowing of the wind, and such very sig- 

 nificant facts as that just noted, will be necessarily slurred 

 over, even by the most sensitive apparatus of this kind. 



However, the more frequent the contacts, the more nearly 

 an exact record of the fluctuations may be measured, and I 

 have, as I have stated, provided that they should be made at 

 every half revolution of the anemometer, that is, as a rule, 

 several times a second. f 



* An example of a very rapid change may be seen on Plate IV, at 12.23 p. 11. 



fHere we may note the error of the common assumption that the ordinary 

 anemometer, however heavy, will, if Motionless, correctlj- measure the velocity 

 of the wind, for the existence of "' vis inertiee " it is now seen, is not indifferent, 

 but plays a most important part where the velocity suffers such great and fre- 

 quent changes as we here see it does, and where the rate at which this inertia is 

 overcome, and this velocity changed, is plainly a function of the density of the 

 fluid, which density we also see reason to suppose itself varies incessantly, and 

 with great rapidity. Though it is probable that no form of barometer in use does 

 justice to the degree of change of this density, owing to this rapidity, we cannot 

 nevertheless, suppose it to exceed certain limits, and we may treat the present 

 records, made with an anemometer of such exceptional lightness, as being com- 

 paratively unaffected by these changes in density, though they exist. 



A.M. Jour. Sci. — Third Series, Vol XLVII. No. -77.— Jan.. ]k<H. 

 4 



