THE PLANE-DROPPER. 33 



Perhaps the most important primary fact exhibited by tliese experiments 

 is that the time of fall for horizontal planes of all shapes is greater as the 

 horizontal velocity increases, and also (as the form of the curves shows) that this 

 retardation in the velocity of falling goes on at an increasing rate with 

 increasing velocities of translation. 



Secondly, we see that those planes whose width from front to back is small 

 in comparison with the length of the advancing edge have a greater time of 

 fall than others. This difference is uniform and progressive from the 6 x 12 

 inch planes to the 18 x 4 inch planes. Expressing this advantage quantitatively, 

 the curves show that the planes having an advancing edge of 6 inches and 

 a width of 12 inches from front to back, when they have a horizontal velocity 

 of 20 meters per second, fall the distance of 4 feet in 0.7 second, while planes 

 of the same area and weight having the advancing edge 18 inches and 4 inches 

 from front to back, when moving with the same velocity, ai-e upheld to such an 

 extent that their time of fall is 2 seconds. This interesting comparative result is 

 also indirectly valuable in giving additional evidence that the largely increased 

 time of fall of the better-shaped planes at the high speeds is not due to the lateral 

 friction of the falling-piece against the frame. The friction with the 6 x 12 inch 

 planes is as great as with any of the others, yet their time of falling is only 

 slightly greater at high speeds than at rest. Attention is called to the fact that 

 at the highest velocity attained in the present series of experiments, 20 meters 

 per second, the curve shows that the time of falling of the 18 x 4 inch planes was 

 increasing very rapidly, so much so as to make it a subject of regret that the 

 slipping of belts prevented experiments at still higher speeds. We may, however, 

 reasonably infer that with a sufficient horizontal velocity, the time of fall may be 

 prolonged to any assigned extent, and that for an infinite velocity of translation, 

 the time of fall will be infinite, or, in other words, that the air will act as a solid 

 support. 



In may be of interest to connect these observations with some partly analogous 

 facts which ai'e more familiar. 



It is frequently observed that a sheet of very thin ice will bear up a skater 

 if he is in rapid motion which would not sustain his weight if he were still ; and 

 even if we neglect the slight difference of specific gravity between water and 

 ice, and suppose the latter to have no differential buoyancy, the rapid skater 

 will still be able to pass safely over ice fhat would not bear his \veight if he 

 were at rest ; for while his mass is the same in both cases, that of the ice called 

 into play in sustaining him is only that corresponding to one unit of area when 

 he is at rest, but to many when he is moving. 



In this form of explanation and illustration the attention is directed only to 

 the action of the air beneath the plane, but in fact the behavior of the air above 



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