218 DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 



of flight, are provided with them. Analogous air-sacs, moreover, are met with in animals 

 never intended to fly ; and of these I may instance the great air-sac occupying the cervical 

 and axillary regions of the Orang-outang, the float or swimming-bladder in fishes, and the 

 pouch communicating with the trachea of the Emu*. 



The same may be said of the hollow bones, — some really admirable fliers, as the Swifts, 

 Martins, and Snipes, having their bones filled with medullary substance, while those of 

 the wingless running birds alluded to have air. Furthermore and finally, a living bird 



o 1Xiii o 



10 lbs. weighs the same when dead, plus a very few grains ; and all know 



what effect a few grains of heated air would have in raising a weight of 10 lbs. from the 



ground. 

 "When we have said that cylinders and hollow chambers increase the area of the insect 



and bird, and that an insect and bird so constructed is stronger, weight for weight, than 



one composed of solid matter, we may dismiss the subject, flight being, as I shall endea- 

 vour to show by-and-by, not so much one of levity as of weight and power properly 

 directed, i. e. weight and power directed on strictly mechanical principles. Those who 

 subscribe to the heated-air theory are of opinion that the air contained in the cavities of 

 insects and birds is so much lighter than the surrounding atmosphere, that it must of 

 necessity contribute materially to flight. I may mention, however, that the quantity of air 

 imprisoned is, to begin with, so infinitesimally small, and the difference in weight which 

 it experiences by increase of temperature so inappreciable, that it ought not to be taken 

 into account by any one endeavouring to solve the difficult and important problem of 

 flight. The Montgolfier or fire-balloons were constructed on the heated-air principle ; 

 but as these have no analogue in nature, and are apparently incapable of improvement, 

 they are mentioned here rather to expose what I regard as a false theory, than as tending 

 to elucidate the true principles of flight. 



Weight indispensable in Flight.— The area of the insect and bird, when the wings are fully 

 expanded, is, with the single exception of the Eats, greater than that of any other class, their 

 weight being proportionally less. It ought, however, never to be forgotten that even the 

 lightest insect or bird is immeasurably heavier than the air, and that there is no fixed rela- 

 tion existing between the weight of body and the expanse of wing in either order. We have 

 thus light-bodied and large-winged insects and birds— as the Butterfly (Plate XIII. fig- 27), 

 Heron(PlateXIV.fig.38), and Albatros,— and others whose bodies are comparatively heavy, 

 while their wings are insignificantly small— as the Sphinx Moth (Plate XIII. fig. 19) and 

 Centaur Beetle (Plate XIII. fig. 16) among insects, and the Grebe, Quail, and Partridge 

 (Plate XIV. fig. 32) among birds. Those apparent inconsistencies in the dimensions of the 

 body and wing are readily explained by the greater muscular development of the heavy- 

 bodied short-winged insects and birds, and the increased power and rapidity with which 



* Nearly allied to this is the great gular pouch of the Bustard. Specimens of the air-sac in the Orang, Emu, and 

 Bustard and likewise of the air-sacs of the Swan and Goose, as prepared by me, may be seen in the Museum of the 

 Royal College of Surgeons of England. The air-sac of the Orang has be'en figured and described by Sandifort in 

 the ' > erhandehngen over de Natuurlyke Geschiedenis der Nederlandsche overseesche Bezittingen,' Leiden, 1840; that 

 of the Emu by Murie, P. Z. 8. 1867, p. 405 ; that of the Bustard by Newton and Cullen, • Ibis/ 1862, p. 107, and 

 186o, p. 143— and also by Flower, P. Z. S. 1865, p. 747. 



