256 DR. PETTIGREW ON THE MECHANISM OF FLIGHT 



4th. Tin feathers of the wing, instead of being closed, as they necessarily are, by a/or- 

 inanl and downward movement, would be inevitably opened, and the integrity of the wing 

 impaired by a downward and backward movement. 



5th. The disposition of the articular surfaces of the wing (particularly that of the 

 shoulder-joint) is such as to facilitate the downward and forward movement, while it 

 in a great incisure prevents the downward and backward one. 



Gth and lastly. If the wing did in reality strike downwards and backicards, a result the 

 converse of that desired would most assuredly be produced, it always happening that 

 an oblique surface which smites the air in a downward and backicard direction (if left 

 to itself) tends to depress the body bearing it. This is proved by the action of free 

 inclined planes, arranged in the form of a screw, upon the air. If a small light screw be 

 placed horizontally in a rest and set free by a rapid rotatory movement, it will be found 

 that, so long as the blades are directed upwards and they are made to strike forwards, 

 or in the direction of the inclination of the blades, the machine will rise. If, however, 

 no change be made in the direction of the blades, and they are made to strike back- 

 tc tnla, i.e. in the direction of the lower margins of the blades, no rising of the machine 

 will occur. Further, the screw if left to itself, invariably acts in opposite directions 

 during its ascent and descent. If a screw, e.g., rises by having its blades inclined in an 

 upward direction from right to left, and by being driven in this direction, it descends by 

 roi tin from left to right, the mere change in the direction of the rotation causing the 

 blades to strike downwards instead of upwards. The effect produced by the downward 

 and backward stroke of an oblique surface upon the air, if its nether margin is directed 

 downwards and backwards, as is the case in the wing of the bird, would consequently be 

 to depress the body of the bird. The reverse of this, as I pointed out at pp. 212, 213, 

 and 1U, holds true of oblique surfaces acting on the water. 



Anakw between the Wing and a Boifs JKfe.-The wing strikes the air precisely 

 as a boy s kite would if it were jerked by its string, the only difference being that 

 he kite is pulled forwards upon the wind by the string and the hand (Plate XV 



tigs, il and tl, c«); r .... _ ° v 



the 



s as an 



hereas in the insect, bat, and bird the wing is pushed forwards 



'"". -:•■ *e ragM rf ,ho body and the power residing i„ the pimon rteelf. 



t TT f <lGSCendS * mak ° S a VMiable ™^ wk the body in addition 



l ; ,;r,f ; r mak r ? h the horizon - s ° that *» «* *«*» *» » «*. „ 



* , J k,JndOT the bod y «*<«* When, moreover, it is being reeovered « 

 £ ESTttT 8 XT Str ° ke ' * 1S *« frM * beW 52* and from 



during pvfpn.inn \uA 1 ,, , V ^ ^"A ° f the Wm g ^ith the horizon 



during extension being gradually diminished. 



The Wing acts as an Elevator, JProveller and QW • t « , . 



, jrfupeuej, ana bustamer, both during Extension 



Since I gave the above explanation of the action of H\p «.««. fT> 



,, .March mn. M, Keda S, Martin haa TllZ _""£ ^T^ ° f "» ^ *-***» ° C <** 



# 



iew to testing the efficacy of this prin 



c.|.le u. artificial flight. It consists of.,, ordiuarv kite with ,„•„ sere, ' ; , S " le eBlc,< * °' ""' prm " 



Ue. of which .rc at right angles to the nlane of L tfa. , , "'" " S U ''P W anl1 cen,ral >" lrt > the 



ith their blades „„ J, I . 1„. .. .. £"" T *? Ue °'" i ,he P laM ° f lesion. The screwsare made to revolve 



A their blades ,„r„ed in a d ward direction, the idea bein* of " TTT ^ *"""■ made «° '""^ 



»" — -* — - -red J the inerda of L^S£%E£ "" *" ^ 



