21(5 DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 



inefficient when the fulcrum is rigid and immobile, it possesses singular advantages when 

 these conditions are reversed, *. e. when the fulcrum, as happens with the air, is elastic 

 and yielding. In this instance a very slight movement at the root of the pinion, or that 

 end of the lever directed towards the body, is followed by an immense sweep of the 

 extremity of the wing, where its elevating and propelling power is greatest,— this arrange- 

 ment ensuring that the large quantity of air necessary for propulsion and support shall 

 be compressed under the most favourable conditions. In this process the weight of the 

 body performs an important part, by acting upon the inclined planes formed by the wings 

 in the direction of the plane of progression. The power and the weight may thus be 

 said to reciprocate, the two sitting, as it were, side by side, and blending their peculiar 

 influences to produce a common result. Prom the mechanical principles on which all 

 winirs are constructed it follows that those insects and birds are endowed with the greatest 



powers of flight whose wings are the longest. The Dragon Ely and Albatros furnish 

 illustrative examples, the former on some occasions dashing along with amazing velocity 

 and wheeling with incredible rapidity; at other times it suddenly checks its headlong 

 career and hovers or fixes itself in the air after the manner of the Kestrel (Plate XV. 



fig. 60) and Humming-birds : the flight of the Albatros is also remarkable, this mag- 

 nificent bird, I am informed on reliable authority, sailing about with apparent uncon- 

 cern for hours together, and rarely deigning to flap its enormous pinions, which stream 

 from its body like ribands to the extent, in some cases, of 7 feet on either side. 



Air-cells and -sacs in Insects and Birds. — The bodies of insects, bats, and birds are con- 

 structed on strictly mechanical principles,— lightness, strength, and durability of frame, 

 and power, rapidity, and precision of action being indispensable. The cylindrical method 

 of construction is consequently carried to an extreme, the bodies and legs of insects dis- 

 playing numerous unoccupied spaces, while the muscles and solid parts are tunnelled in 

 every direction by innumerable air-tubes, which communicate with the surrounding 

 medium by a series of apertures termed spiracles. 



A somewhat similar disposition of parts is met with in birds, these being in many cases 

 furnished not only with hollow bones, but also (especially the aquatic ones) with a liberal 

 supply of air-sacs. They are likewise provided with a dense covering of feathers or down, 

 which adds greatly to their bulk without materially increasing their weight. Their bodies, 

 moreover, in not a few instances, particularly in birds of prey, are more or less flattened. 

 The air-sacs are weU seen in the Swan, Goose, and Duck ; and I have on several occasions 

 minutely examined them with a view to determine their extent and function. In two of 

 the specimens which I injected, the material employed had found its way not only into those 

 usually described, but also into others which ramify in the substance of the muscles parti- 

 cularly the pectorals. Whether, however, this was due to rupture and extravasation I 

 have not since had an opportunity of ascertaining. No satisfactory explanation of the 

 purpose served by these air-sacs has, I regret to say, been yet tendered. According to Sap- 

 pey*, who has devoted a large share of attention to the subject, they consist of a membrane 



* Sappey enumerates fifteen air-sacs :-the thoracic, situated at the lower part of the neck, behind the sternum I 



two cervical, whir.h run fhp wlinlo lonn-tV. n e *\* n l 1.11..1 ■ .... . . •— 



0/ posterior diaphragmatic; and two pairs of 



>/ 



occur 



