328 The Mechanics of Soaring. [ April, 
“That the value of the force is twelve pounds moving the surface 
on the air with uniform velocity, which we will suppose to be five 
feet per second. Under these conditions it is obvious that all the 
force is employed in condensing and otherwise disturbing air, so _ 
that it is doing work on the air at the rate of twelve pounds each 
second. It is also obvious that while uniform velocity occurs 
there is a moving equilibrium between the surface acted on by the 
force and the reacting air, and that any additional force, however 
small, applied to the surface at any angle to its own plane would 
disturb this balance, and either accelerate or retard its velocity. 
If, however, the additional force be applied in its plane, lateral 
motion would occur without changing the equilibrium. In the 
lateral motion no air is disturbed, nor driven out of the way, nor 
condensed, excepting what is caused by smooth skin, or surface 
friction, and this is so very small as to elude all my attempts to find 
its value. One pound applied in the plane of the surface would 
doubtless drive it at the rate of 1000 feet per second. We will 
an suppose, however, that one pound will drive it from 4 towards @ 
at the rate of thirty feet per second. As it is moving towards d 
| at the rate of five feet per second, it will pass neither towards d 
„nor a, but on the diagonal å e, the resulting path, the parallelo- 
-gram being thirty feet long and five feet wide. 
_ Note the character of the equilibrium between the surface and 
air. The total force is flowing around the edges of the surface in 
_ the shape of condensed and otherwise disturbed air. If the hand 
were placed under the surface with an upward push, precisely as 
much resistance as would be given to the hand would be taken 
_ from the air, and the velocity retarded. If scales. were applied to 
$ 
7 
moving the index and precisely that much less would be working 
on air, and the velocity of the surface would be again retarded. 
_ If the surface met with sufficient impediment to stop its motion, 
_ alt the force would be resisting the impediment and none doing 
= work on air, and velocity would cease. k 
_ ~ Itis evident that at uniform velocity the force has been trans- 
_ ferred. It is in the air tension, and not in the surface, for to be also 
there, would necessitate its creation out of nothing, There being 
_ then, no force in the surface, it could not antagonize any resist- 
ance offered to it, and this is in fact its condition as we have seen- 
While its velocity remained uniform it. would obey any: impulse: 
the surface and a pull towards c given, part of the force would benn 
