4 
movements of the animal with the organs of motion merely expanded. 
And there is a corresponding variety of forces concerned in produ- 
cing these results, such as that of gravitation, muscular power, accel- 
eration, and the force of the winds. And in the process of what 
may be called flight proper, contrary to the popular belief and the 
results of all previous investigations, I have discovered that the first 
named power exerts the chief influence. And in order to show clearly 
the difference between my own theory founded upon this principle, 
and that of the current philosophy on this subject, I will here 
present the outlines of the latter hypothesis, and then proceed to 
give a true explanation of the phenomenon of flight, contained in 
two somewhat distinct parts, one of which will be illustrated by a 
diagram. 
The vague and unsatisfactory theory of the flight of birds found in 
the books, is based upon the supposition that gravitation, or the 
weight of the animal, is the great obstacle to be overcome in this pro- 
cess ; and in order to meet this difficulty, the authors of it were 
driven to the further supposition, that the bird is capable of exerting 
through the pectoral muscles an enormous amount of strength for so 
small an animal ; and having thus inferred the means of attaining the 
proper elevation, it was finally imagined, that, by imparting to the 
wings an oblique movement, the effect of which must be both to sus- 
tain the weight and propel the body, they jumped to the conclusion 
that these were the means, and this the philosophical solution of 
flight proper ! 
And in accounting for certain curious evolutions in the air, per- 
formed by some of the large winged species only — such as that of 
ascending by gyratory movements, and of sailing far away on straight 
lines, without any other effort than that of merely expanding the 
wings — naturalists profess to have discovered a nameless gas which, 
they say, is secreted within the hollow ends of the quillfeathers ; that 
all birds in the process of breathing have the peculiar power of infla- 
ting with air a large internal membraneous duct extending through- 
out the length of the spine, and also the cavities of all their bones, 
and of exhausting both at pleasure ; and each of these marvelous 
means is supposed by this theory to play an important part in pro- 
ducing those graceful movements of the bird on the motionless wing. 
Such is a brief but truthful statement of the chief elements of the 
