section of the circle, and losing nothing in rapidly sweeping over the 
Opposite side, the animal at last attains the desired elevation. And then 
he usually sails away towards the point from whence the wind blows, 
progressing comparatively slow under the influence of gravity alone 
on a slightly descending line, the time of his fall being prolonged by 
the current of air which he is stemming playing under his wings ; but 
still, it is evident that he must soon come to the earth unless he re- 
sumes his gyratory movements, or be met by a wind of sufficient force 
to entirely arrest his progress for an instant, and thus find himself 
poised " upon airy nothing," aS we often see it happen. 
And finally, it will be perceived tlftit if the foregoing theory be 
true, the flight of birds, as it occurs in connection with both the mov- 
ing and motionless wings, is not comprehended in the three laws of 
motion of Newton ; and therefore it cannot be explained by reference 
to the principles and circumstances which govern bodies that are im- 
pressed with motion by a single impulse of either natural or mechani- 
cal forces. 
Unlike all other motions, the most important process of this one, 
as exhibited in connection with the flapping wings, is efl'ected by the 
constant influence of two opposing for'ces, the one being extraneous 
and natural and the other muscular, and of course derived from and 
accompanying the moving body, resulting in the origination of a 
third agency, not to say power, and with its influence the production 
of the usual swift aerial progression of birds on a horizontal line. 
And the other marked feature of it, as exhibited on the motionless 
wing in connection with gyratory movements, is the result of two 
extraneous forces, the one constant and the other casual, and which 
at successive intervals unitedly act upon the soaring animal in one 
direction, and thereby impart to his body considerable velocity ; while 
the latter, at corresponding periods, alternately moves in unison with 
the two controlling forces, and in turn encounters the combined in- 
fluence of both in the opposite direction ; and thus the spiral elevation 
of the large winged birds is gradually efl'ected. 
And these remarkable characteristics, doubtless, have hitherto pre- 
vented the true philosophical solution of this standing, or more pro- 
perly speaking, flying puzzle to the minds of all men, both before 
and since Solomon's day, in whose proverbs it is said (among other 
things since found out) that " the way of an eagle in the air is a 
mystery." 
