6 
the particular direction he designs going : and that, so soon as the 
effect of such effort to elevate the weight ceases, the animal instantly 
begins to gravitate towards the earth, not perpendicularly, but in a 
slanting forward direction, while at the same time, by a gentle exer- 
tion of his organs of motion, as seen in short excursions, he is enabled 
to maintain the desired height whilst moving through the air with 
his usual velocity. If, however, the object be to rise high above the 
earth, as in preparing for long migrations, the wings are exerted with 
greater celerity and power, the head and fore part of the body being 
slightly raised, and for a short time he struggles upward with a rapid 
motion. And in thus ascending, it may be fairly inferred, I think, 
that the body acquires a considerable degree of momentum, which, 
while it imparts uniformity of progress upon the line of ascent, also 
relieves the annual from the labor of sustaining his full weight after 
the first few vigorous impulses of his wings. But still, it is equally 
probable, that the power employed in this process, exceeds the amount 
expended in flight proper, as much as that which must be exerted by 
a person in climbing a steep mountain transcends the amount required 
in the usual manner of walking on a level surface. And, at any rate, 
it is evident, that in all such eases the labor is so great that both 
men and animals soon become exhausted, as is proved by every day 
experience with reference to ourselves, and as is equally certain, from 
the strongest rational inference, in regard to birds. 
The desired elevation having been thus attained by the migrating 
birds, let us now consider the process of flight on a horizontal line, as 
exhibited in their motion whilst traversing large districts of country, 
in passing and repassing to and from the places which they frequent 
in winter and summer. After slightly relaxing the power by which 
one of these animals raised himself, and adjusting his body in a level 
position on the line of motion, he instantly begins to descend on a 
certain angle formed below that line — resulting in each case from the 
comparative area of wing and weight of body— -under the increasing 
influence of gravity, while the equally constant action of the organs 
of motion effectually counteract that tendency ; and in the conflict 
between these two forces, the body incidentally moves in a forward 
direction, and whilst thus progressing on a horizontal line soon acquires 
a considerable degree of accelerated motion, which, resulting from and 
combining with the influence of gravity, impels it onward in a still 
