72 



RAPIDITY OF FLIGHT. 



rose, flying to the point B. Knowing 



to make any material alteration in the result, though if greater 



accuracy is required, 

 it may be obtained 

 by the usual mode of 

 measuring triangles- 

 Thus, let b c be the 

 height of the tower, 

 and a the point from 

 whence the Starling 

 the height of the tower 

 and the distance a c, we have to calculate a b, which is 

 easily learned, since by the well-known problem of Euclid, 

 a b 2 = a c 2 + b c 2 ; by extracting the square root, we therefore 

 find the exact length of a b. 



It was by an application of this simple rule that the flight 

 of an Eagle was ascertained to be little short of one hundred 

 and forty miles an hour. The bird was seen hastening on its 

 way over a valley in the Pyrenees, and the number of seconds 

 was observed which elapsed between its passing from the sum- 

 mit of one high point till it reached the brow of a mountain 

 on the other side, the space between which was known by 

 reference to a good map, in which the distances were well laid 

 down. Those who ride over commons of fine turf may often 

 have witnessed a quickness of flight, probably not much in- 

 ferior to that of the Eagle ; for they will, even at their fullest 

 speed on the fleetest horse, have seen Swallows skimming in all 

 directions, pursuing the small insects which the horse puts up 

 in its course over the grass, sometimes leisurely keeping at an 

 equal pace, then shooting ahead, and not unfrequently actually 

 flying round the rider in wide circles, with an ease and facility 

 betraying neither effort nor labour in so doing. 



The flight of the common Swallow has been computed at 90 

 miles, that of the Swift has been conjectured to be nearly 180 

 miles per hour. We can scarcely, indeed, calculate or limit the 

 speed which can be produced by the effort of a wing's vibra- 

 tions. That a small insect can with ease accomplish forty or 

 fifty miles an hour, and probably much more, we know to be 



