1^2 ON AERIAL NAVIGATION. 



Hence its whole velocity must be 42'5 feet per second. 



Velocity of the Should the same surface be wafted downward, like a wins, 

 oblique stroko. • , ^, , . , , . ,. , , . , „ , 



will the hinder edge inclined upward in an ans;^]e of about 



50°'40' to the current, it will overtake it at a velocity of 3*5 



feet per second ; and as a slight unknown angle of resistance 



generates a pound pressure per square foot at this velocity, 



probably a waft of little more than 4 feet per second would 



produce this effect; one tenth part of which would be the 



Ii* advantaga propelling power. The advantage in favour of this mode of 



^"^" ^- application, compared with the former, is rather more than 



ten to one. 



Diffici;Uiis m jj^ combining the general principles of aerial navigation 



mecnaiiisni to • ,. i i • , t v> , ■ 



be overcome, for the practice ot the art many mechanical dithculties pre- 

 sent themselves, which require a considerable course of 

 skilfully applied experiments, before they can be overcome, 

 but enough But to a certain extent the air has already been made navi- 

 to promise ul- g^^'^ J ^^^ ^^ one, who has seen the steadiness with which 

 timate success, weights to the amount of ten stone (including four stone, 

 the weight of the machine) hover in the air, can doubt of 

 the ultimate accomplishment of this object. 

 . Firstflifficultys The first impediment I shall take notice of is the great 



want of initial . « ^i^ -i ..j -^j 



Telocity. proportion or power, that must be exerted previous to the 



machiae's acquiring that velocity, which gives support upon 

 the principle of the inclined plane; together with the total 

 want of all support during the return of any surface used 

 like a wing. Many birds, and particularly water fowl, run 

 and flap their wings for several yards before they can gain 

 support from the air. The swift (hirundo apus Lin. J is not 

 able to elevate itself from level ground. The inconvenience 

 under consideration arises from very different causes in these 

 two instances.- — The supporting surface of most swimming 

 birds does not exceed the ratio of -,Vths of a square foot to 

 every pound of their weight: the swift, though it scarcely 

 weighs an ounce, measmes eighteen inches in extent of 

 wing. The want of surface in the one case, and the incon- 

 venient length of wing in the other, oblige these birds to aid 

 the comrnencement of their flight by other expedients; yet 

 they can both fly with great power, when they have acquired 

 their full velocity. 

 Scconrl: great A second difficulty in aerial navigation arises from the 

 exieiit of lever, ^ 



great 



