BIRD-FLYING. 
55 
when it does, the velocity of the falling body will ceafe to 
increafe. It follows, therefore, that when a body falls 
through the atmofphere, its rate of acceleration is con¬ 
tinually diminifhed; and there is a limit beyond which 
the velocity of its fall cannot increafe ; this limit being 
determined by that velocity at which the relifting force of 
the air will become equivalent to the gravity of the body. 
“ As the relifting force of the air, other things being 
the fame, increafes with the magnitude of the furface pre- 
fented in the direction of the motion, it is evidently polfible 
fo to adapt the fhape of the falling body, that any required 
limit may be imprelfed upon the velocity of its defcent. It is 
upon this principle that parachutes have been conftrudted. 
“ When a body attached to a parachute is difengaged 
from a balloon, its defcent is at first accelerated, but very 
foon becomes uniform, and as it approaches the earth, the 
air becoming more and more denfe, the refiftance on that 
account increafes, and the fall becomes ftill more retarded.” 
—Lardner’s Natural Philofophy , book iii. chap. 9. 
It is manifeft, without argument, that an afcenlional 
current of air eftablilhed underneath the parachute, with 
a force equivalent to the fall of the body, would have the 
fame retarding effedt as the defcent of the parachute 
through the air: alfo that an increafe of the afcenlional 
force beyond this point would fupport the parachute in 
oppofition to the force of gravitation, and eftablilh a 
fulcrum of locomotion. 
