Internal Work of the Wind, 435 



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 h 10 m 18 s rose within 10 seconds to a velocity of 33 miles 

 an hour, and within 10 seconds more fell to its initial speed. 

 It then arose 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 ohserve, 

 passing through 18 notahle maxima and as many notable 

 minima, the average interval from a maximum to a minimum 

 being a little over 10 seconds, and the average change of 

 velocity in this time being about 10 miles an hour. In the 

 lower left-hand corner of Plate VII. is given a conventional 

 representation of these fluctuations in which this average 

 period and amplitude is used as a type. The above are facts, 

 the counterpart of which may be noted by any one adopting 

 the means the writer has employed. It is hardly necessary 

 to observe, that almost innumerable minor maxima and 

 minima presented themselves, which the drawing cannot 

 depict. 



In order to ensure clearness of perception, the reader will 

 bear in mind that the diagram does not represent the velocities 

 which obtained coincidently, along the length of two miles of 

 wind represented, nor the changes in velocity experienced by 

 a single moving particle during the interval, but that it is a 

 picture of the velocities which were in this wind at the suc- 

 cessive instants of its passing the fixed anemometer ; which 

 velocities, indeed, were probably nearly the same for a few 

 seconds before and after registry, but which incessantly passed 

 into, and were replaced by, others, in a continuous flow of 

 change. But although the observations do not show the 

 actual changes of velocity which any given particle experiences 

 in any assigned interval, these fluctuations cannot be materially 

 different in character from those which are observed at a fixed 

 point, and are shown in the diagram. It may, perhaps, still 

 further aid us in fixing our ideas, to consider two material 

 particles as starting at the same time over this two-mile 

 course : the one moving with the uniform velocity of 22 # 6 

 miles an hour (33 feet per second), which is the average 

 velocity of the wind as observed for the interval between 

 12 h 10 m 18 s and 12 h 15 m 45 s on February 4 ; the other, during 

 the same interval, having the continuously changing velocities 

 actually indicated by the light anemometer, as shown on 

 Plate V II. Their positions at any time may, if desired, be 

 conveniently represented in a diagram where the abscissa of 



