IN RELATION TO AERONAUTICS. 327 



varying according to the density of the atmosphere and the shape, extent, and 

 velocity of the body acting upon it. While, therefore, ahnost no impediment 

 is offered to the progress of an animal in motion, it is often exceedingly difficult 

 to compress the air with sufficient rapidity and energy to convert it into a suitable 

 fulcrum for securing the onward impetus. This arises from the fact that bodies 

 moving in this medium experience the minimum of resistance and occasion the 

 maximum of displacement. Another and very obvious difficulty is traceable to 

 the great disparity in the weight of air as compared with any known solid, (this 

 in the case of water being nearly as 1,000 to 1,) and the consequent want of 

 buoving or sustaining power which that disparity necessitates. To meet these 

 peculiarities the insect and bird are furnished with extensive surfaces in the 

 shape of pinions or wings, which they can apply with singular velocity and 

 power at various angles, or by alternate slow and sudden movements, to obtain 

 the necessary degree of resistance and non-resistance. Their bodies, moreover, 

 are constructed on strictly mechanical principles — lightness, strength, and dura- 

 bility 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 displaying numerous unoccupied spaces, wdiile 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 also provided with a 

 dense covering of feathers or down, which adds greatly to their bulk without 

 materially increasing their weight. The air-sacs are well seen in the swan, 

 goose, and duck ; and I have in several instances carefully examined them with 

 a view to determining their extent and function. They appear to me to be con- 

 nected with the function of respiration, a view advocated by Hunter in 1774, 

 and within the last year or so by Drosier, of Camluidge. That they have 

 nothing whatever to do with flight is proved by the fact that some excellent 

 flyers — take the bats e. g. — are destitute of them, while the wingless running 

 birds, such as the ostrich, and apteryx, which are incapable of flight, are pro- 

 vided with them. The same may be said of the hollow bones, some really 

 admirable flyers, as the swallows, martins, and snipes, having their bones filled 

 with medullary substance, while the bones of the running wingless birds alluded 

 to are filled with air. Ftn-tliermore, and finally, a living bird weighing 10 

 pounds weighs the same when dead minus a very few grains ; and all know 

 what effect a few grains of heated air would have in raising a weight of 10 

 pounds 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 endeavor to show by-and-by, not so much one of weight as of 

 power properly directed, i. e. power directed on strictly mechanical principles. 

 Those who subscribe to the heated-air theory are of opinion that the air con- 

 tained in the cavities of insects and birds is so luuch lighter than the surround- 

 ing atmosphere, that it must of necessity contribute materially to flight ; but 

 the quantity of air imi)risoned is, to begin with, so infinitesimally small and the 

 difference in weight which it experiences by increase of temperature so inajipre- 

 ciable, that it ought not to be taken into account by any one endeavoring to 

 solve the difficult and important problem of flight. The Montgolficr 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 need not 

 detain us at this stage of the inquiry. The area of the insect and bird when 

 the wings are fully expanded is, with the single exception of the bats, greater 



