434 Prof. S. P. Langley on the 



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 slowing of the wind, and 

 such very significant facts as that just noted will be neces- 

 sarily 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 *. 



I now invite the reader's attention to the actual records of 

 rapid changes that take place in the wind's velocity, selecting 

 as an illustration the first 5^ minutes of the diagram plotted 

 on Plate VII. 



The heavy line through points A, B, and C represents the 

 ordinary record of the wind's velocity as obtained from a 

 standard Weather Bureau anemometer during the observations 

 recording the passage of two miles of wind. The velocity, 

 which was, at the beginning of the interval considered, nearly 

 23 miles an hour, fell during the course of the first mile to a 

 little over 20 miles an hour. This is the ordinary anemometric 

 record of the wind at such elevations as this (47 metres) above 

 the earth's surface, where it is free from the immediate 

 vicinity of disturbing irregularities, and where it is popularly 

 supposed to move with occasional variation in direction, as the 

 weather-cock indeed indicates, but with such nearly uniform 

 movement that its rate of advance is, during any such brief 

 time as two or three minutes, under ordinary circumstances, 

 approximately uniform. This, then, may be called the " wind ; " 

 that is, the conventional " wind " of treatises upon aerody- 

 namics, where its aspect as a practically continuous flow is 



* Here we may note the error of the common assumption that the 

 ordinary anemometer, however heavy, will, if frictionless, correctly 

 measure the velocity of the wind, for the existence of u vis inertiae " it is 

 now seen, is not indifferent, but plays a most important part where the 

 velocity suffers such great and frequent 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 comparatively unaffected by these changes in density, 

 though they exist. 



