60 S. P. Langley — Internal Work of the Wind. 



I have supposed, however, the wind pulsation to cease at the 

 end of a certain brief period, and, to fix our ideas, let us sup- 

 pose this period to be five seconds. At this moment the period 

 of calms begin, and now let the plane, which is supposed to have 

 reached the point B, change its inclination about a horizontal 

 axis to that shown in the diagram, falling at first nearly verti- 

 cally, with its edge on the line of its descent, so as to acquire 

 speed, and this speed, acquired, by constantly changing its 

 angle, glide down the curve B o C, so that the plane shall be 

 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 contra- 

 diction 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 experi- 

 mented upon by him. 



]STow, 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 latter's presenting the 

 proper angle, but this time under more favorable circumstances, 

 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 begins, it falls with varying 

 inclination, to the lowest position E (which may be higher 

 than 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 alternation 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 hori- 

 zontal, 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 alter- 

 nate 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 



