ON AERrAL NAVIGATION. iQg 



air, and keeps tight in all weuthere. When bracings are well 

 applied, they make the poles, to which they are attached, 

 btai- endwise. The hollovy form of the quill in birds is a 

 very admirable structure for lightness combined with 

 strength, where external bracings cannot be had ; a tube * 



being the best applicatiou of matter to resist as a lever ; but 

 the principle of bracing is so effectual, that, if- properly ap- 

 plied, it will abundantly make up for the clumsiness of hu- 

 man invention in other respects; and should we combine 

 both these principles, and give diagonal btacing to the tu- 

 bular bamboo cane, surfaces might be constructed with a 

 greater degree of strength and lightness, than any made use 

 of in the wings of birds. 



The surface of a heron's wing is in the ratio of 7 square Proportion* of 

 x> ^ . in J- 11 • ^- • weight to saj^ 



teet to a pound. Jlence, accordmg to this proportmn, awing f^j-e^nt^g 



of 54 square feet would weigh about 7| pounds: on the con- wiags of birds, 

 trary the wings of water fowl are so much heavier, that a sur- 

 face of 54 square feet, according to their structure,will weigh 

 18^ lb. 1 have in these instances quoted nearly the ex- 

 tremecases amongst British birds; the wing I have described 

 may therefore be considered as nearly of the same weight in 

 proportion to its bulk as that of most birds. 



Another principle exhibited in this wing is that of the ResistaDo* 



poles being couched within the cloth, so as to avoid resis- ''y°'"s^ ^J 



'^. . . . / placiDg tbe 



tance. This is accomplished by the convexity of the frame, nbs remote 



and the excessive lightness of the cloth. The poles are not ^'°"^ '^^ *^«"* 

 allowed to form the edge of the wing, excepting at the ex- 

 treme point of the bow, where it is very thin, and also ob- 

 lique to the current. The thick part of this pole is pur- 

 posely conveyed considerably within the edge; In birds, a 

 membrane covered with feathers is stretched before the 

 thick part of the bone of the wing, in a similar manner, ^nd 

 for the same purpose. The edge of the surface is thus re- 

 duced to the thickness of a small cord, that is sown to the 

 cloth, and gives out loops whenever any fastening is requir- 

 ed. The upright shaft is the only part that opposes much 

 direct resistance to the current, and this is obviated in a 

 preat degree by a flat oval shape, having its longest axis 



parallel to the current. 



nu ■ • . u- c .v.- - -.u .11 Strengthofthe 



ine joint or hinge or this yiiag acts with great firmness, hinge. 



in 



