10 
required in this process, in elucidation of the position assumed in my 
third postulate. 
The great velocity of the bird on the wing is entirely due to the 
influence of gravity and acceleration, as already partially explained > 
and the weight of the animal is involved in his motion to a far greater 
extent, perhaps, than has ever been suspected by any of those per- 
sons who have heretofore attempted to explain the phenomenon of 
flight ; and, after long observation and mature reflection on all the 
matters connected with this interesting subject, I can confidently lay 
down the following rule as the basis of my remarks upon the point 
under consideration, namely : The smaller the wings ^ compared with 
the weight of the animal, (within a certain limit) and the more fre- 
quent their impulses {to the extent of at least three revolutions per 
second of time,) the greater the velocity, and the smaller the amount 
of power required in traversing the air on a horizontal line. 
The truth of this position may be readily established, I think, by 
a brief explanation of the organisms, respectively, of the eagle, wild- 
goose and duck, which birds furnish the best examples of swift and 
long continued flight. 
The wings of these animals, and those of the duck especially, are 
comparatively small and symmetrically formed, and consequently they 
are enabled to fly at the rate of about one himdred miles per hour 
without being much afi"ected, either in the course pursued or the de- 
gree of their velocity, by the winds. 
And these, like all other birds, traverse the atmosphere on the 
wing with a uniform motion, being incapable of either increasing or 
diminishing their velocity whilst continuing on the same plane of 
elevation. For it may be seen by any one who will take the pains ta 
observe the flight of birds generally, that if the action of their organs 
of motion be reduced by fatigue, or from any other cause, below the 
proper number of strokes to sustain them on the line of flight, they 
instantly begin to descend) and on the other hand, if increased above 
the required rate, they merely ascend. Now, it may be safely inferred 
from this^ that the amount of power employed in this process is uni* 
form, and that it is graduated in each case by the relative area of 
wing and weight of the animal. What the exact amount is in any 
case would be extremely dijB&cult to ascertain ; but in view of all the 
circumstances connected with the production of this motion, I give it 
as my opinion that, among all birds similarly organized with those 
