64 THE ZOOLOGIST. 
the centre of the earth, precisely as it does in all other cases, and 
the reason that the body manifesting it does not get lower is 
because something is pushing up against the under surface, just 
hard enough to balance the weight. It may be hard to follow all 
the peculiarities of the disturbances going on under the bird, but 
it is certain that they serve to hold it up. They are mainly 
condensations of air upon which the body is falling, and are equal 
to ten pounds in each foot of air passing to the rear. This ten 
pounds of force is moving at the rate of thirty feet per second, as 
we assumed at the start, and it follows that an amount capable of 
holding up 300 pounds each second is passing the rear edge of 
the bird’s wings, and is wasted in falling to the tension of the 
surrounding air. . 
But this is not all the force of disturbance which passes to 
the rear. The reactions against air-resistance also go there. 
These, like the others, consist in condensations, accelerations, 
and deflections. According to the law of the composition and 
resolution 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 revolving planes. Supposing in this case the height to be 
one-fifth the base, there would be 360 pounds of force passing the 
bird’s wing each second. Gravity puts in 300 pounds, and gets 
itself supported in doing it. The weight is thus balanced; 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 consequently 
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 phenome- 
non will be the weight of the bird. It only requires one-sixth of 
the whole force on hand to do it. If this can be utilised 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 
