314 ON AERIAL LOCOMOTION. 



miicb greater tbau for auy other periormauce of flight. The body of the 

 bird at the time is nearly vertical. The wings uphold the weight, not 

 by striking vertically downwards upon the air, but as inclined surfaces 

 reciprocating horizontally like a screw, but wanting in its continuous 

 rotation in one direction, and in consequence of the loss arising from 

 rapid alternations of motions, the power required for the flight will ex- 

 ceed that specified in the screw experiment before quoted, viz, 3 horse- 

 power for every 100 pounds raised. 



We have here an example of the exertion of enormous animal force 

 expended in flight, necessary for the peculiar habits of the bird, and 

 for obtaining its food ; but in the other extreme, in large heavy birds, 

 whose wings are merely required for the i)urposes of migration or loco- 

 motion, flight is obtained with the least possible degree of power, and 

 this condition can only be commanded byarnpid straight-forward course 

 through the air. 



The sustaining power obtained in flight must depend upon certain 

 laws of action and re-action between relative weights; the w'eight of a 

 bird, balanced, or finding an abutment, against the fixed inertia of a 

 far greater weight of air, continuously brought into action in a given 

 time. This condition is secured, not by extensive surface, but by great 

 length of wing, which, in forward motion, takes a support upon a wide 

 stratum of air, extending transversely to the line of direction. 



The pelican, for example, has wings extending out 10 feet. If the 

 limits of motion imparted to the substratum of air, acted upon by the 

 incline of the wing, be assumed as 1 foot iu thickness, and the velocity 

 of flight as 30 miles per hour, or 2,640 feet per niinute, the stratum of 

 air passed over in this time will weigh nearly 1 ton, or one hundred times 

 the weight of the body of the bird, thus giving such an enormous sup- 

 porting power that the comparatively small weight of the bird has but 

 little efl'ect in deflecting the heavy length of the stratum downwards, 

 and therefore the higher the velocity of flight the less the amount of 

 " slip " or power wasted in compensation for descent. 



As noticed at the commencement of this paper, large birds may be 

 observed to skim close above smooth water without ruffiing the surface, 

 showing that during rapid flight the air does not give way beneath them, 

 but approximates towards a solid support. 



In all inclined surfaces, moving rapidly through the air, the whole 

 sustaining power approaches toward the front edge; and, in order to 

 exemplify the inutility of surface alone, without proportionate length 

 of wing, take a plane 10 feet long by 2 broad, impelled with the nar- 

 row end forward, the first 12 or 15 inches will be as efficient at a high 

 speed in supporting a weight as the entire following portion of the 

 plane which may be cut off, thus reducing the effective wing-area of a 

 pelican, arranged in this direction, to the totally inadequate equivalent 

 of 2i square feet. 



One of tlie most perfect natural examples of easy and long-sustained 



