THE PLANE-DROPPER. 47 



for only a very slight depth ; .so that for the planes 4 inches apart, at the average 

 speeds, the stratum of air disturbed during its passage over it, is, at any rate, less 

 than 4 inches thick. In other words, the plane is sustained by the compression 

 and elasticity of an air layer not deeper than this, which we may treat, for all our 

 present purposes, as resting on a solid support less than four inches below the 

 plane. (The reader is again reminded that this sustenance is also partly due to 

 the action of the air above the plane.) 



Summing up the results obtained with the plane-dropper, we have determined : 



1. The relative times of falling a distance of 4 feet (l m .22) that obtain for 

 differently shaped hut horizontally disposed planes moving with different hori- 

 zontal velocities, showing quantitatively the primary fact that the time of fall is 

 an increasing function of the velocity of lateral movement, 



2. The varying velocities of translation at which planes of given size and 

 weight, but of different shapes, will be sustained by the air when inclined at 

 different angles. 



3. The maximum proximity at which successive planes can be set one above 

 the other in order to give a supporting power proportional to their surface. 



4. A first approximation to the initial amplitude of the wave motion origi- 

 nated by a plane passing horizontally or at a small angle through the air with a 

 considerable velocity. 



5. The approximate resistance to advance of a wind-plane at soaring speeds, 

 and (by computation) the work necessary to be expended in overcoming this 

 resistance. 



These experimentally show that the higher horizontal speeds are maintained 

 with less expenditure of power than lower ones, and the quantitative experiments 

 by which these results are established are, so far as I am aware, new, and I 

 believe have a most immediate bearing on the solution of the problem of artificial 



flight. 



I may add that these experiments with the horizontal plane, when properly 

 executed, give results of a character to forcibly impress the spectator; for, since 

 there is no inclination, there is no visible component of pressure to prolong the 

 fall, yet the plane nevertheless visibly behaves as if nearly deprived of its weight. 

 The "pair of 18 x 4 inch planes, for instance, T V of an inch thick and weighing 

 464 grammes, has a specific gravity of about 1,660 times that of air ; yet while the 

 retardation due to the still air in the direct fall is but 20 9 .03, that due to the same 

 air in strictly lateral motion is 1*.50— a most noteworthy result in its bearing on 

 the use in mechanical flight that may be derived from a property of the air much 

 utilized by nature, but hitherto almost wholly neglected in this connection by 

 man — its inertia. 



