974 DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 



or 



ginning 



are with a, 01 Diagram 



wing in an opposite direction, as seen 



down 



The 



wing, 



therefore, during its descent, describes a twisting, sinnous, or wave-like track, which is dif- 

 ferently, but equally faithfully, represented at a a of fig. 59. The track described by the 



or, what is the same, the blur or impression produced on the eye by its continuous and 



rapid action, is, in fact, spiral in its nature; and if the space traversed by the wing were 



represented by a solid, it would take the form of the blade of a screw-propeller, as shown at 



ca, da, of fig. 52. 



Fig. 59. Blur or impression produced on the eye by the rapidly oscillating wing of the Blow-Fly when the 



insect is progressing at a high speed. Seen from above and from the side. In this case the 

 sinuous path or wave-like track (a a') described by the wing during its descent and ascent is more 

 apparent than in fig. 58, which is intended more especially to show the rotatory action of the 

 pinion. The present figure represents the wing towards the termination of the down stroke (a) 

 and the beginning of the up one (a'), the wing during its continuous action ascending and falling 



m a series ot cur\ 



as represented at diagram 6, p. 233. Here 



(fig. 59 a a) corresponds to the curve described by the extremity of the ordinary screw propeller, 

 as a comparison of a a' of the present figure with the corresponding letters of fig. 52 will 

 read i ly show. While, therefore, the extremity of the wing and the wing itself is describing a series 



acefnl cur\ 



v. ^vv^^wwvxj \*fj »F c*x UO ttllU. UU >> xxVV ClXLLO, U HJA/Ax -ixcio 



the effect of equalizing the motion, the body of the insect is carried steadily along a given line, 



its iinSltirvn l^rnTirr ilnm-trd -*>-» t A »rn «* 1-^i. il i t i ., . . _ . 



nation of the up and down stroke. The 



midway between that assumed by the wing: at the be 



k is owing to the 



wi 



Note 



smoothness 



furnished by the wings when in motion. 

 Fig. 60. Kestrel or Windhover, hunting (after 



Duke of Argyll 



engraving by Mr. Wolff in His Grace the 



x,uKe or Argyll s work, 'The Eeign of Law'). Here the bird is represented as balancing 

 or poising itself m the air. This feat is accomnl i s h Pd W *h» «,-„«. ~M— - .„•;*» of 



short strokes, the wmg in no instance descending qnite to the level of the body. 



made bv tlm win or witl-i +I10 i^™„ • i ., . _ _ * 



The 



reversing slightly. 



some 



from a parachute 



over mchned almost perpendicularly, the tail, which is spread out, being directed downwards 

 Z , "a" ."I aSS1StmS * hC Wi " gS " ™ inta »™g *c equilibrium of the trunk. In some 



instances thn hnl n-nnn ia o^ ««^ j..i__ • . . , _ * 



m 



the ule „ '# 1 7 « • *° beh ° ld * he KeStrel in this P-M™ *® ™* «*» *«* 

 tne pleasure attorded bv the siaTi* Tiir, u • i . -, 



glona st eUat ar , lm) are likewise clpablc of 2EK? f *"-**** M ° th *"** 

 def. anterior or thick ™ZL f 5 ta6 T^ ^ d « te ™ s ** » •** «™ uatttlCS - 



primary 



margin, formed 



Fig. 61. Blur 



seen oXfg of fig. 62 {vide arrows). 



position of the head and anterior portion oftkbiri 



iary {C ) leathers. At d the wind escapes after having been 

 under surface of the wing. The groove in Question is well 



■&• •"*« g 



in maintaining the elevated 



of the bird 



Seen from the left side. Shows how *L ! F UBCmanon ot ^ ^it wmg ot tne u»~ 



spiral track, a a>, durin- its d^ ^ 2 T" " WS °" * ^ ^ ml ***** * 

 i , , uuiin a its aescent and ascent. It also *W- C +w l\- j <r„^r» «tnkc, 



also shows that the 



