1885.] The Problem of the Soaring Bird. T169 
of forces they bear the same relation to the vertical disturbances 
which the height of, the incline bears to the base, or, in other 
words, they are to each other as the angle of inclination of the 
resolving planes. Supposing in this case the height to be one- 
fifth the base, there would be 360 pounds of force passing the 
rear of the bird’s wing each second. Gravity puts in 300 pounds 
and gets itself supported in doing it. The weight is thus bal- 
anced ; but we are employing an external force of sixty pounds 
to push the body on the air. By the law of the action of elastic 
fluids under pressure, when the condensed air passes the rear 
edge of the wing surfaces it expands in all directions, and conse- 
quently upwards and forwards on that edge. If sixty pounds of 
the whole 360 expanding is thus thrown forwards, it will balance 
air resistance, and the total power to produce the soaring phe- 
nomenon will be the weight of the bird? It only requires one- 
sixth of the whole force on hand to do it. Ifthis can be utilized 
by wasting the other five-sixths the task is accomplished. There 
would be waste in eddies and side currents, so that in reality 
there would be less than the total force of disturbance passing to 
the rear. Allowance may be freely made for all wastage and 
sufficient will remain to perform the desired service. Experiment 
shows that in very critical tests the result can be attained without 
the rear expansion. It may be held that each molecule of air as 
it is struck by the plane is, to some extent, carried with it, as 
well as condensed, while those below are at rest. Thus a rotary 
motion may be supposed to take place on the entire lower sur- 
face, when the air-resistance factor would be neutralized by the 
excess of gravity in each molecule instead of at the rear edge. 
It has been objected that this is a disguised form of “ perpetual 
motion.” Remember that there is a great difference between 
heaping absurdity on a thing and finding absurdity in it. It is 
urged that gravity can only do work by the fall of the body man- 
ifesting it, and that in this case it does not fall; that there is no 
sacrifice of its energy of position, and hence it cannot do work. 
This is true of actions in which the earth, or anything fast 
to or supported by it, is a part, and the gravitating body another 
part ; and it is true in the sense that the word “fall” means get- 
ting nearer the earth. But this is not true with soaring nor with 
allied phenomena, and I will illustrate the matter by an example. 
Suppose that our hypothetical bird rests in the air in a horizontal 
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