50 /S. J*. Langley — Internal Work of the Wind. 



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 III. 



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 meters) 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 maybe called the "wind," 

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

 namics, 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 secs 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 observe, 

 passing through 18 notable 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 velo- 

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

 left hand corner of Plate III, is given a conventional repre- 

 sentation of these fluctuations in which this average period and 

 amplitude is used as a type. The above are facts, the counter- 

 part 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 insure clearness of perception, the reader will 

 bear in mind 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 succes- 



