24 
of the latter to be about twenty times greater than that of the 
former. 
Hence, it is certain that in this condition of things any movement 
of those organs by the bird, designed either to produce or maintain 
the horizontal motion, would only result in obstructing his progress ; 
for they could not be turned in any direction appropriate to that ob- 
ject, without instantly coming in contact with the impinging atmos- 
phere ; and the extent of any such change from their natural position 
upon, and necessary relation to, the body would be the exact measure 
of the deleterious eflPect thereby produced on the motion of the 
animal. 
Now in Tiew of all that has been said relative to the action of the 
wings, and the eflfects thereby produced, it is very evident that the 
popular belief that the horizontal motion of birds in the air is the re- 
sult of an oblique movement of those organs is not only ill-founded, 
but is likewise clearly at fault with the nature of propulnion, as ex- 
emplified in all cases of muscular and mechanical locomotion, in con- 
formity with the general application of the second law of motion of 
Newton. According to this law, all bodies acted upon by a single 
f orce move in the line of the impulse by which they are changed from 
one place or position to another : and it is quite certain that every 
species of motion, except that of our present inquiry, whether pro- 
duced by muscular or other power, is referable to this general princi- 
ple — that propulsion is the mode by which the various processes are 
carried on ; and that organs and machinery are the means employed 
in all cases of prolonged systematic operations of this kind, acting 
either in a constant rotatory manner, or by a succession of alternate 
movements, according to the cii'cumstances of each particular case. 
The truth of this position is too plain to require any argument in its 
support ; and I will here only make a statement of the relation in- 
variably existing between the organs in the one case, and the apparata 
in the other, and the results of their action, common to the processes 
of both muscular and mechanical motion, by way of illustration. 
The organs of motion of terrestial animals occupy a position under- 
neath the body, from the very necessity of the case, and the exertion 
of them in the process of locomotion, from a similar necessity, results 
in the propulsion of the animal in a straight line with the application 
of the power employed, or in that of the movement of the organs — the 
