2G4 DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 



rliynchus, and the various kinds of swimming birds may be cited. The principle here 

 adverted to i- likewise traceable in the movements of Clio borealis, the Rotifera, and 

 J}Icihiscp, the mushroom-shaped disk of the latter opening and closing, and affording 

 perhaps the simplest illustration of the efficacy of the inclined plane that can be given. 



III. In fishes, the hands and feet, as is well known, are absent. The body, how- 

 over, is much elongated, and in some cases greatly flattened, the vertebral column 

 possessing that freedom of motion which enables the animal to apply the hinder 

 half of the trunk to the water very much as a boatman would an oar in sculling. 

 By a series of lashing, flail-like strokes, in which the tail is made to move vigorously 

 from side to side or is briskly twisted and untwisted, the degree of resistance and non- 

 resistance requisite for propulsion in this medium is obtained. While the tail is oscil- 

 latin g from side to side, it is made to rotate slightly on its axis, so as to diminish the 

 amount of surface exposed during flexion or when it is drawn slowly from the mesial 

 line, as happens in the back stroke— the flat of the tail being applied 'during extension, 

 or when the tail suddenly springs towards the mesial line, as happens in the effective 

 stroke. The tail of the fish acts as an inclined plane in two directions : but that plane 

 acts the most ell', etivelv which is directed towards the axis of the fish, because of the 

 increased velocity with which it is urged, and the greater area presented by it. The 

 amount of surface applied to the water by the fish is greater than in the amphibia and 



birds, and nenee its greater speed. The sea-mammals, as the Whale, Dugong, Manatee, 

 tnd Porpoise, swim precisely as the fish does, with this difference, that the tail strikes 



vert ,cal y instead of horizontally. The Walrus and Seal are exceptional in this respect. 



IV. in the Flymgfish, provision is made for the animal leaving the water for a brief 

 interval It is consequently furnished, in addition to its swimming-tail, with greatly 



S^kK^ T WMCh ' Whm thG ° reatUre takeS t0 the ™> ™ ^ raised above 

 the body nil they make a certain angle with it. Asthefins, whiletheyare being extended, 



o th^ ir , 5? r i urned in au ° uwd directi ° n m ^ *«** « «U of w, 



Ze fl 1 J the ^r^ aDd Cl ° Sed ' tUrned in ™ ds > and l0 — d wL they 



Z^uJLT T n thG air fl '° m *— * after the — r of a kite. Here 



>v 2 Z ^ ■ non-ieaurtanee necessary to sustain the fish in the air is obtained 



of the my^Z f T ^ VariaWc *** the incli - d V^es and the weight 



*S %Ztt C0 T? uting t0 this — In L 



Dower this hp,nc, m«L i , ° e re " ard ed as an independent moving 



fun on m 2^ Ti\ tlie 0WiqUe ^^ ****** * the wings in con- 



junejon mth the power expended by the animal-the latter bein~ by this simple com- 

 bination, conserved to a remarkable extent w*- w «. uem ^' °y ims slm P le com 

 li-ament or smW * wi»Vl 1Kawe f tent - ^ eight, therefore, assisted by the elastic 

 ugameni or spimg , which recovers the win* in flexion i« +~ \* » ^ ^ 



nical expedient resorted to by natnre in sunrl! 1 ******* ™ the mGC ^' 



Without this, flight would be of tiZl'Z7f7t *, thG Gff0rtS ° f aU ^ thiDgS - 



short duration, laboured, and uncertain, and the almost 



height, as erery one knows, is the sole moving power in th P 1 I 

 the movements produced by the descent of the leads In wat I " the P endulu m being used merely to regulate 



and it is worthy of remark that the mechanician ^hns .eized' 2' 7* ° f ^^ iS tWn Up °" * V iral **"'* 

 Ployed in the animal kingdom. P ' &n ln g enio »sly utilized, two forces largely em- 



