On the various modes of flight in relation to aeronautics / by James Bell Pettigrew.

Date:
[1867]
Catalogue details

Licence: Public Domain Mark

Credit: On the various modes of flight in relation to aeronautics / by James Bell Pettigrew. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

    4/14 (page 4)
    variouH 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 princijdes: lightness, strejigth, 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 disjdaying numerous unoccupied spaces, while the muscles and solid parts are tunnelled in every direction by innumer- able 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 arc also provided with a dense covering of feathers or down, which adds gi’catly 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 deter- mining their extent and function. They appear to mo to be connected with the function of respiration, a view advocated by Hunter in 1774, and mthin the last year or so by Drosier, of Cambridge. 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 provided 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 rimning wingless birds alluded to are filled with air. Furthermore, and finally, a living bird weighing lOlbs. weighs the same when dead minus a very few grains; and all know what efiect a few grains of heated air would have in raising a weight of lOlbs. from the ground. When we liavo said that cylinders and hollow chambers increase the area of the insect and bird, and that an insect and bird so con- structed is stronger, weight for weight, than one composed of solid matter, we may dismiss the subject, flight being, as I shall endeavour 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 much lighter than the surrounding atmosphere, that it must of necessity contribute mate- rially to flight, but the quantity of au- imprisoned, is to begin with, so infinitesimally small aud the diiforenco in weight which it expe- riences by increase of temperature so inappreciable, that it ought not to be taken into account by any one endeavouring to solve the difficidt and important pi-olilom of flight. The Moutgidflor or fire baloons were constructed on the heated-air princijjle, but as these have no analogue in nature and are aj)i)arently incaj)able of improvement, they need not detain us at this stage of the inquiry. The ai-ea of the insect