•'JH THE MECHANICAL FUNCTIONS. 



wards, the greatest part of the effect produced by- 

 its action is to move the body forwards. Birds of 

 prey have a great obliquity of wing, and are con- 

 sequently better formed for horizontal progressive 

 motion, which is what they chiefly practise in pur- 

 suing their prey, than for a rapid perpendicular 

 ascent. Those birds, on the contrary, which rise 

 to great heights in a direction nearly vertical, such 

 as the Quail and the Lark, have the wings so dis- 

 posed as so strike directly downwards, without any 

 obliquity whatsoever. For the same reason, birds 

 rise better against the wind, which, acting upon the 

 oblique surface presented by the wings during their 

 flexion, contributes to the ascent of the body, on 



second of time, or 1380 beats per minute ! — a conclusion, of which 

 the simplest observation of the actual motion of the wings is suffi- 

 cient to show the fallacy. 



The following is among the most curious of the results of Navier's 

 analytical researches. Assuming that a labourer when employed in 

 turning a winch during eight hours a day, produces a mechanic 

 effect equivalent to the raising of a weight of 6 kilogrammes, 

 (or 13 avoirdupois pounds), to the height of one metre, (or 3.28 

 feet), in each second of time, which is about 43 pounds raised to 

 the height of one English foot) ; and assuming the weight of the 

 man so working to be 70 kilogrammes, (or 154 pounds), this me- 

 chanic force, if employed to raise himself, would carry him to a 

 height of 0.086 metres (or 3.366 inches) in each second of time. 

 But the action exerted during each second by a bird, when only ho- 

 vering in the air, is sufficient to raise its own weight to a height of 8 

 metres, (or 313 inches), which is 93 times greater than the height to 

 which the corresponding exertion of a man would enable him to 

 attiiin. It follows as a consequence, that if the human structure 

 were such as to enable a man to employ his muscular power in the 

 way best calculated to raise his body in the air, and that also he had 

 the means of expending the whole of the force which he exerts during 

 an ordinary day's labour of eight hours, in as short a space of time as 

 he pleased, the concentrated effect of this labour would be no more 

 than to sustain him in the air during five minutes. Ibid. p. Ixxii. 



