214 DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 



Prom what has heen stated it will he evident that the wing acts very differently in and 

 out of the water ; and this is a point deserving of attention, the more especially as it seems 

 to have hitherto escaped observation. In the water the wing strikes downicards and 

 backwards, and acts as an auxiliary of the foot ; whereas in the air it strikes downwards and 

 forwards. The oblique surfaces, spiral or otherwise, presented by animals to the water and 

 air are therefore made to act in opposite directions, as far as the down and up strokes are 

 concerned. This is owing to the greater density of water as compared with the 

 the former supporting or nearly supporting the animal moving upon or in it, the latter 

 permitting the creature to fall through it in a downward direction between the ascent and 

 descent of the wing. To counteract the tendency of the bird in motion to fall in a down- 

 ward and forward direction, the stroke is delivered in the direction in which falling would 

 naturally occur, the kite-like action of the wing, and the rapidity with which it is moved 

 causing the mass of the bird to pursue a more or less horizontal direction. I offer this 

 explanation of the action of the wing in and out of the water after repeated and careful 

 observation in tame and wild birds, and, as I am aware, in opposition to all previous 

 writers on the subject*. The movement of the wing of the bird in true flight is fully 

 explained at pages 224-227. 



Before proceeding to a consideration of the graceful and, in some respects, mysterious 

 evolutions of the denizens of the air, and the far-stretching pinions by which they are 

 produced, it may not be out of place to say a few words in recapitulation regarding the 

 extent and nature of the surfaces by which progression is secured on land and on or in 

 the water. This is the more necessary, as the travelling-surfaces employed by animals in 

 walking and swimming bear a certain, if not a fixed, relation to those employed by Bats, 

 Inserts and Birds in flying. On looking back, we are at once struck with the fact, re- 

 markable m some respects, that the travelling-surfaces, whether feet, flippers, fins 

 pinions are as a rule, increased in proportion to the tenuity of the medium on which they 

 are destined o operate This is well seen in Plate XIL, where the subjects are expressly 

 arranged to illustrate the increase in question. In the Ox (fig. 1) we behold a ponderous 

 body slender ex tremi ieS , and unusually small feet. The feet are slightly expanded in 



i^L^^r» » *J* Omithorhynchus (%• 3) The Lvellin 



or 



area 



mented in the Seal (fig. 4), Penguin (fi 6) Sea . B J r fi ^ 



omi2 la " /r ( f 8) - a hUge s ™^' " added to the feet-the Ml be 



ZoL Z im n emitieS (anteriOT) d "^S> i" the Manatee (fig. 0) and 



wl/Salr TT ^ the &h (fiS ' n ' »*» » ot «* «"» tail but * 

 lower half of the body is actively engaged in natation. Turnip from the water to the air, 

 we observe a remarkable modification in +K„ 1, i , ° \ 



these enabling the creaturTto take 1 ^ ^^ & ™ ° f the ^V"*" <** ^ 



ing in like manner fromlh ^h tolT ^ - d Se ^ - P-^.inions. Tom- 

 expansions of the Plying ft,?^ ? ^^ the immense tegumentary 



i => <* on imte XIII. fig. 13) and Galeopithecus (Plate XIII. 



s 



sel 



th . . - 



opinions 



Macgill 



* acts in ordinary flight see foot-notes to p. 253. 



