1886.] The Mechanics of Soaring. 3 329 
derived from incident forces directed upon it, being powerless to 
resist them. 
So far all is evident, but the surface would not fill the condi- 
tions of soaring. Its motion is not at right angles to the force, 
but inclined thereto, and to accomplish the desired result its path 
must be perpendicular to the force. 
If we now tilt the surface on one of its long edges, 4, say for 
two inches, or one in six, it would no longer follow the direction 
of the force, but slant sideways towards this edge. Following 
the law of the composition of forces, one-sixth of the force, or 
two pounds, will now drive the surface sideways, and there being 
nothing but trifling skin friction to oppose it, it will obey the im- 
pulse. But if we apply a force of two pounds to the edge to 
. balance this thrust, equilibrium will once more occur and the sur- 
: face will move in the path of the force. Experimentally, far less 
than two pounds is required for this rest, as a considerable incline 
is reached before the sideways thrust is developed, the confused 
air currents under the surface obscuring the movement. 
Note that this abutting force is of the nature of pressure. It 
is a static feature devoid of motion. It is a rest for the tilted sur- 
face to lean against. 
It cannot be denied that we now have a case of equilibrium once 
more. With the two-pound rest a perfect balance results and the : 
surface moves in the path of the force. All the force is disturbing i 
air, while the surface moves with uniform velocity. Af that speed 
it is unable to offer the least resistance to any further force which 
may be opposed to it. At this point the answer to a single ques- 
tion will be decisive. If it be in the affirmative the position is 
Secure; if in the negative, my explanation of soaring is wrong, _ z 
_ the birds are still in the air waiting solution. oo 
_ The question is this; Will the tilted surface, supplied with the 
rest of two pounds and moving with uniform velocity, obey the ae . 
impulse of an external force, applied in its own plane, with equal 
facility in any direction? gee 
It certainly is evident that the only resistance to such a force — 
_ arises from skin friction in whatever way the motion is made. — If 
ou there be any other from whence comes it? It cannot come from: 
the original force, for this is fully employed. It cannot come 
from the two-pound rest, as this is already balanced by the side= 
A of the reacting air. This does not vn pih the 
PA 
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