Internal Work of the Wind. 445 



horizontal velocity of the wind, and consequently opposing 

 less resistance to it, and therefore moving more and more 

 laterally, and rising less and less, at every successive instant. 



If the wind continued indefinitely, the plane would ulti- 

 mately take up its velocity, and finally, of course, fall, when 

 this inertia ceased to oppose resistance to the wind's advance. 

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

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

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

 period of cairn begins, and now let the plane, which is sup- 

 posed to have reached the point B, change its inclination 

 about a horizontal axis to that shown in the diagram, falling 

 at first nearly vertically, 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&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 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, having reached C, at the end of the H\e 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 favourable 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 CD, 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 0), 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 horizontal, or a slow 

 and nearly vertical course, or anything between. 



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



