25 
strength being expended in one direction, while the body moves in 
the opposite course. And this is the universal condition of the loco- 
motion of all terrestrial and aquatic animals. 
And so in the process of mechanical locomotion, whether produced 
on the surfaces of either the earth or the water — the action of the ap- 
parata designed for that purpose invariably resulting in the propul- 
sion of the body in a line corresponding with the direction of the 
power applied to it. The submerged propeller of the steam-vessel 
forms no exception to this constant relation betwixt the application 
of the force and the motion produced, notwithstanding its peculiar 
form and position ; for the principle is the same as in that of the ordi- 
nary side wheels of such vessels, the power being expended in a back- 
ward direction, while the ship or boat progresses forward. But not so 
in the process of flight. The direction in which the wings are exerted 
bears no such relation to the horizontal motion of birds as that which 
has just been shown to exist in the process of locomotion among all 
terrestrial animals, properly so called, and by mechanical means. In 
this case it is unquestionably true that the power is expended in im- 
parting a simple vertical movement to those organs,, and that, too, at 
right angles instead of being in a right line with the motion of the 
animal ; and therefore it is next to impossible for the latter to be the 
product of the former. Hence it is clear that the aerial progression 
of birds does not result from an oblique movement of the wings. And 
in addition to all that has been said in regard to this matter, I venture 
the remark that there cannot be found within the whole range of 
zoology a single animal whose self-progression is effected by a com- 
pound or twofold action of a single set of organs of motion. Nature 
has yet to exhibit such a startling anomaly as that, and the genius of 
Art would blush at the bare suggestion of the possibility of such a pro- 
cess through the application of the laws of mechanics, either in mov- 
ing stationary machinery, or in imparting progressive motion to sepa- 
rate bodies. And it does seem to me that nothing in the way of 
philosophical pretensions could be more strange and unsatisfactory 
than the teaching of the popular theory on this subject. For the- 
generally received notion, that birds propel themselves through the 
air by the action of their wings, is involved in the double absurdity 
of a body moving in a direction forming a right angle with the line of 
the force applied to it, and also with a much greater velocity within 
