THE INTEKXAI, WORK OF THE WIND. 21 



tangential to it at every point of its descending advance. At the end of five 

 seconds of calm it has reached the position C, near the lowest point of its descent, 

 which there is no contradiction to known mechanical laws in supposing may be 

 higher than A, and which, in fact, according to the most accurate data the writer 

 can gather, is higher in the case of the above period, and in the case of such an 

 actual plane as has been experimented upon by him. 



Now r , having reached C, at the end of the five seconds' calm, if the wind blow 

 in the same direction and velocity as before, it will again elevate the plane, on the 

 hitter's presenting the proper angle, but this time under more favorable circum- 

 stances, for, at this time, the plane is already in motion in a direction opposed to 

 that of the wind, and is already higher than it was in its original position A. Its 

 course, therefore, will be nearly that along the curve C D, during all which time it 

 maintains the original angle a, or one very slightly less. Arrived at D, and at the 

 instant when the calm begius, it falls, with varying inclination, to the lowest posi- 

 tion E (which may be higher thau C), which it attains at the end of the five seconds 

 of calm, then rises again (still nearly at the angle a) to a higher position, and so 

 on ; the alternations of directions of motion, at the end of each pulsation, growing 

 less and less sharp, and the path finally taking the character of a sinuous curve. 

 We have here assumed that the plane goes against the wind and rises at the same 

 time, in order to illustrate that this is possible, though either alternative may 

 be employed, and the plane, in theory at least, may maintain on the whole a 

 rapid and nearly horizontal, or a slow and nearly vertical course, or anything 

 between.* 



It is not meant, either, that the alternations which would be observed in 

 nature are as sharp as those here represented, which are intentionally exaggerated ; 

 while in all which has just preceded, by an equally intentional exaggeration of the 

 normal action, the wind-pulsations have been supposed to alternate with absolute 

 calm. This being understood, it is scarcely necessary to point out that if the calm 

 is not absolute, but if there are simply frequent successive winds or pulsations of 

 wind of considerably differing velocity (such as the anemometer observations show, 

 are realized in nature), that the same general effect will obtain, though we are not 



* See the very interesting account {Aeronautical Annual, No. 2, p. 66) by Mr. Chanute of the 

 successive steps by which sea-gulls were actually observed to get in motion without flapping. 

 The above a priori reasoning reads almost like a description of Mr. Chanute's subsequent ob- 



servation. 



