10 TIIK IMKKNAI. WORK OF THE WIND. 



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 the} 

 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 5h minutes of 

 the diagram plotted on Plate III. 



The heavy line through points A, B, ami (', 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 disturb- 

 ing 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, dining 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 aero- 

 dynamics, where its aspect as a practically continuous flow is alone considered. 

 When, however, we turn to the record made with the specially light anemometer, 

 at every second, of this same wind, we find an entirely different state of things. 

 The wind starting with the velocity of 23 miles an hour, at 12 hrs. 10 mins. 18 

 sees., rose within 10 seconds to a velocity of 33 miles an hour, and within 10 

 seconds more fell to its initial speed. It then rose within 30 seconds to a velocity 

 of 36 miles an hour, and so on, with alternate risings and fallings, at one time 

 actually stopping; and, as the reader may easily observe, passing through 18 

 notable maxima and as many notable minima, the average interval from a maximum 

 to a minimum being a little over in seconds, and the average change of velocity in 

 this time being about 10 miles an hour. In the lower left-hand corner of Plate III. 



* Here we may note the error of the common assumption that the ordinary anemometer, how- 

 ever heavy, will, if frictionless, correctly measure the velocity of the wind, for the existence of "vis 

 inertias, " it is now seen, is not indifferent, but plays a most important part where the velocity suffers 

 such great and frequent changes as we In re si e it does, and where the rate .'t 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 prob- 



i hit no form of barometer in use dots 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. 



