THE [NTERNAX WiilIK OF THE WIND. 11 



is given a conventional representation of these lluet. nations, 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 insure clearness of perception, the reader will bear in mind that 

 the diagram does not represent the velocities which obtained coincidentally, along 

 the length of two miles of wind represented, nor the changes in velocity experi- 

 enced by a single moving particle during the given interval, but that it is a picture 

 of the velocities which were in this wind at the successive instants of its passing 

 the fixed anemometer, which velocities, indeed, were probablj 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 obser- 

 vations do not show the actual changes of velocity which any given particle 

 experiences in any assigued 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 oue moving with the uniform velocity of 22.6 miles an hour (33 feet per 

 second), which is the average velocity of this wind as observed for the interval 

 between 12 hrs. 10 mins. 18 sees., and 12 hrs. 15 mins. 45 sees., on February 4; 

 the other, during the same interval, having the continuously changing velocities 

 actually indicated by the light anemometer as shown on Plate III. Their positions 

 at any time may, if desired, be conveniently represented in a diagram, where the 

 abscissa of any point represents the elapsed time in seconds, and the ordinates 

 show the distance, in feet, of the material particle from the starting-point. The 

 path of the first particle will thus be represented by a straight line, while the path 

 of the second particle will be an irregularly curved line, at one. time .above, and at 

 another time below, the mean straight line just described, but terminating in coinci- 

 dence with it at the end of the interval. If, now, all the particles in two miles of 

 wind were simultaneously accelerated and retarded in the same way as this second 

 particle, that is, if the wind were an inelastic fluid, and moved like a solid cylinder. 

 the velocities recorded by the anemometer would be identical with those that 

 obtained along the whole region specified. But the actual circumstances must evi- 

 dently be far different from this, since the air is an elastic and nearly perfect fluid, 

 subject to condensation and rarefaction. Hence the successive velocities of any given 

 particle (which are in reality the resultant of incessant changes in all directions), 

 must be conceived as evanescent, taking on something like the sequence recorded 



