ON AKRiAr. NAVIGATION'. iQ^ 



great extent of lever, which is constantly operating aguinst 

 the first mover, in consequence of the distance of llie cen- 

 tre of support in large surfaces, if applied in the aranner of 

 wings. 



A third and general obstacle is the mechanical skill re- Third: to 



quired to unite great extenbion of surface with strength at;d *"' '"' i^'^e 



1- 1 , p , .• 1 • ,. - ?trt-net:i and • 



hghtness ot structure; at the same tune havmg a hrm and lijjhtness with 



steady movement in its working parts, without exposing uu- cKtentof sur- 



necessary obstacles to the resistance of the air. The first of ji^p first ob- 



these obstacles, that have been enumerated, operates much s'^fi^- '^^ss felt 



i' 11 • . • I • -• 1 ■ , on a small 



more powertully against aerial navigation upon a large scale, ^^.^jg^ 



than against birds; because the small extent of their wings 

 obliges them to employ a very rapid succession of strokes, 

 in order to acquire that velocity which will give support; 

 and during the small interval of the return of the wiug, their 

 weight is still rising, as in a leap, by the impulse of one 

 stroke, till it is again aided by another. The hirge surfaces 

 that aerial navigation will probably require, though neces- 

 sarily moved with the same velocit}% will have a proportion- 

 ably longer duration both of the beat and return of the 

 wing; and hence a greater descent will take place during 

 the latter action, than can be overcome by the former. 

 * There appears to be several ways of obviating this diffi- Modes of ob- 

 culty. There may be two surfaces, each capable of sus- '''^*'"S '' °" * 

 taining the v/eight, and placed one above the other, having ^ *" ^ 

 such a construction as to work up and down in opposition 

 when they are moved, so that one is always ready to descend, 

 the moment the other ceases. These surfaces may be so 

 made, by a valvelike structure, as to give ny opposition in 

 rising up, and only to resist in descent. 



The action may be considered either oblique, as in rota- 

 tive flyers; alternately so, without any up and down wafr, 

 as in the engine 1 have ascribed to Mr. Degen; by means of 

 a number of small wings in lieu of large ones, upon the 

 principle of the flight of birds, with small intervals of time 

 between each waft; and lastly by making use of light 

 wheels to preserve the propelling power both of the beat and 

 the return of the wings, till it accumulates sufficientiv to 

 elevate the machine, upon the principle of those birds which 

 M '2 run 



