164. 



ON AERIAr- NAVIGATION. 



run themselves up. This action might be aided by making 



choice oi' a descending ground like the swil't. 



A mancanuse "With regard to another part of the first obstacle I have 

 great exertion .,.,,, • ,. , i j 



fcraihorttime. ''"^"t'oned, viz. the absolute quantity or power demandeo. 



being so much greater at first than when the full velocity 

 has been acquired; it may be observed, that, in the case of 

 human muscular strength being made use of, a man can 

 exert, for a few seconds, a surprising degree of force. He 

 can run up stairs, for instance, with a velocity of from 6 to 

 8 feet perpendicular height per second, without any danger- 

 ous etfort; here the muscles of his legs only are in action; 

 but, for the sake of makino- a moderate statement, suppose 

 that with the activity of his arms and body, in addition to 

 that of his legs, he is equal t© raising his weight 8 feet per 

 second; if in this case he weighs 11 stone, or 154 pounds, 

 he will be exerting, for the time, an energy equal to more 

 than the ordinary force of two of Messrs. Boulton and 

 Watt's steam horses ; and certainly more than twelve men 

 can bestow upon their constant labour. 



If expansive tirst movers be made use of, they may be so 

 constructed, as to be capable of doing more than their con- 

 stant work ; or their power may be made to accumulate for 

 a few moments by the formation of a vacuum, or the con- 

 densation of air, so that these expedients may restore at on^ 

 time, in addition to the working of the engine, that which 

 they had previously absorbed from it. 



With regard to the second obstacle in the way of aerial 

 navigation, viz. the length of leverage to which large wing- 

 like surfaces are exposed, it may be observed, that, being a 

 constant and invariable quality, arising from the degree of 

 support such surfaces give, estimated at their centres of re- 

 sistance, it may be balanced by any elastic agent, that is so 

 placed as to oppose it. Let A and B, PI. iV, fig. 1, be two wings 

 of an aerial vehicle in the act of skimming; then half the 

 weight of the vessel is supported from the centre of resist- 

 ance of each wing; as represented by the arrows under 

 them. If the shorter ends of these levers be connected by 

 cords to the string of a bow C, of sufficient powei to ba- 

 lance the weight of the machine at the points A and B, then 



the 



Other expe- 

 dients. 



^Jeihod of 

 obviating the 

 »2Cond diffi- 

 culty, 



by a counter- 

 balancing 

 spring, 



