o 



234 DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 



the plane of progression is greatest at the middle of the down stroke (#), when the 

 pinion makes an angle of 30° or thereabouts with the horizon*, and least during 

 the middle of the up stroke (d), when the wing presents a narrow or cutting edge 

 to the wind. This is occasioned by the gradual increase in the angle made by the 



with the horizon during the first part of the down stroke (/), and its gradual 

 diminution during the second part (g). The angle is diminished during the second 

 part of the down stroke to prepare the wing for commencing the up or back stroke, 

 the pinion being gradually rotated off the wind to present less and less resistance. 

 I'll is result is facilitated during the back stroke by the concavity of the wing travelling 

 along t he convexity of the curve d, this being the direction in which the wing is raised pre- 

 ] oratory to a second down stroke. The down and up strokes are consequently compound 

 movements, — the termination of the down stroke, as has been explained, embracing the be- 

 ginning of the up stroke, the termination of the up stroke, on the other hand, including 

 the beginning of the down stroke. This is necessary in order that the down and up 

 stroke^ may glide into each other in such a manner as to prevent jerking and unneces- 

 sary retardation,— the angle made by the wing during the first part of the down stroke 

 being increased to support and propel the insect, and decreased during the second part 

 to diminish the friction caused by the wing itself, while it does not interfere with its 



sustaining-power. 



A ""logy between the icing m motion and the sounding of sonorous bodies, 8fC.— What 

 has just been said of the wing of the insect holds equally true of the pinion of the bat and 

 bird ; and it is a remarkable circumstance that the undulation or wave made by the wing 

 when the insect and bird are fixed or hovering before an object, and when they are 

 progressing, corresponds in a marked manner with the track described by the stationary 

 and progressive waves in fluids f, and likewise with the waves of sound \. This coinci- 

 dence would seem to argue an intimate relation between the instrument and the medium 

 on which it is destined to operate-the wing acting in those very curves into which the 

 atmosphere is naturally thrown in the transmission of sound, in order, as appears to me, 

 .^T" tl f. maximum of , Progression with the minimum of slip. Can it be that the 

 .... . reciprocate, and that animal bodies are made to impress 



the inanimate in precisely the same manner as the inanimate impress each other ? This 

 much seems certain :-The wind communicates to the water similar impulses to those 

 commumcated to it by the fish in swimming; and the wing in its vibrations impinges 

 7.Z a T* ™ ° ^ ^ would. The extremities of quadrupeds, moreover, 



wen m Tt 1 7 " 1 G ^ When Walkin ^ and ™^ ^ - that one great law 

 would seem to determine the eonr«iP nf +Vi« ^„~ ± • ^i • , 1 



., „ llo , , * , , fte COUrse ot tne msect "i the air, the fish in the water, and 



the quadruped on the land. 



inmate world 



* The down stroke is most effective towards the miiMlp «f t\, a j i .- „ 

 made bv the •»* .M. ifc.-l._i • , undulation for two reasons :-first, because the angle 



made bv the wing with the horizon is greatest at this period- and semnrfW \. n- • , , 



level with the hnHv an A tu^e i j . i ' secon aly, because the wings are then on the same 



t £** of Natura! Hut M . on E , tricity , Magnetism> ^ ^^ ^ ^ ^^ 



pp. 36G-7. 



t Op. cit. pp. 378, 379, 380. 



