326 ON THE VARIOUS MODES OF FLIGHT 



tlie ribs affording tlic necessar}' degree of su]iport in the dragon or flying lizard, 

 the anterior and posterior extremities in the flying lemur, flying cat, and bat. 



Although no lizard is at present known to fly, there can be little doubt that 

 the extinct pterodactyles, which are intermediate between the lizards and croco- 

 diles, were possessed of this power. 



The bat is interesting as being the only mammal at present enjoying the priv- 

 ilege of flight ; it is likewise instructive, as showing that flight may be attained 

 without the aid of hollow bones and air-sacs, by purely muscular eftbrts and by 

 the mere contraction and dilatation of a continuous membrane. 



If we now direct our attention to the water we And that the amount of sur- 

 face engaged in locomotion greatly exceeds that in the amphibia. The flsh fur- 

 nishes the best example. 



In it the lower half of the body and the broadly-expanded tail are applied to 

 the water very much as an oar is in sculling. The sea-mammals, as the whale, 

 dugong, manatee, and porpoise, swim in precisely the same manner as the fish, 

 with this diflerence, that the tail strikes from above downwards, or vertically 

 instead of horizontally, or from side to side. The seal is exceptional in this 

 respect. 



The animals which furnish the connecting link l)ctween the water and the air 

 are the flying fishes on the one hand, and the diving birds on the other; the 

 fonner sustaining themselves for considerable intervals in the air by means of 

 their enormous pectoral fins, the latter using their wings for flying above and 

 beneath the water, as occasion demands. 



I have carefully examined the relations, stmcture, and action of the fins in 

 the flying-fish, and am of opinion that they act as true pinions ; their inadequate 

 dimensions only pircventing them from sustaining the fish for an indefinite period 

 in the air, at all events so long as they remain moist. They operate upon the 

 air from beneath, after the manner of a kite or spiralifer, and in so doing, lever 

 the animal upwards and forwards. 



If they did not act as true pinions within certain limits it is difficult and indeed 

 impossible to understand how such small creatures could obtain the momentum 

 necessary to project them a distance of 200 or more 3'ards, and that sometimes 

 at an elevation of 20 feet above the water. 



In birds which fly indiscriminately above and beneath the water, the wing is 

 general!}' provided with stirter feathers than usual, and reduced to a minimum as 

 regards size. In subaqueous flight the wings may act by themselves, as in the 

 guillemots, or in conjunction with the feet, as in the grebes ; but in either case 

 it is the back or convex surface of the wing which gives the effective stroke, the 

 W'ing in such birds as the great auk, which are incapable of flight, being for this 

 purpose twisted completely round, in order that its concave surface, which takes 

 a better hold of the water, may be directed backwards. 



The wing, therefore, operates very diflerently in and out of the water. 



In the water it acts as an auxiliary of the foot, and both strike backwards 

 and downwards. 



In the air, on the contrary, it strikes downwards and forwards, and this is a 

 point deserving of attention, as showing that the oblique surfaces presented by 

 animals to the water and air are "made to act in opposite directions. This is 

 owing to the greater density of the water as compared with the air ; the former 

 supporting or nearly sui)porting the animal acting upon it ; the latter permitting 

 the animal to fall through it in a downward direction. 



But to come to the subject more particularly in hand, viz: 



Flight in its relation to Aeronautics. — The atmosphere, because of its great 

 tenuity, mobility, and comparative imponderability, presents little resistance to 

 bodies passing through it at low velocity. If, however, the speed be greatly 

 increased, the action of even an ordinary cane is sufficient to elicit a recoil. 



This comes of the action and reaction of matter, the resistance experienced 



