82 ON aeriAl navigatiok. 



Stability of a subject move familiar, let us recollect, that iti a bout, sftitn-- 

 ruing upon water, its stability or stiffiiess depends, in ge- 

 ueral terms, upon the tceig/it and distance from the centre, 

 of the sectiou elevated above the water, bj^ any given heel 

 of the boat, on one side; and on the bnl/c, and its distance 

 from the centre, which is immersed below the water, on the 

 otiier side; the combined enckavour of the one to fall, and 

 of the other to swim, produces the desired effect in a well 

 cotistructed boat. The centre of gravity of the boat being- 

 more or less below the centre of suspension is an additional 

 cause of its stability. 



Effect of a ra- Let us now examine tlie effect of a parachute represented 



rachi.te, as ,^ ^ jj p- , pj^ ^ \y\^qi^ \^ ]^,^^ hvL'\e(\ into the po- 



cCnimoiiiy ... 



made, sition a h, the side a is become perpendicular to the cur- 



rent, created by the descent, and therefore resists with its 

 greatest power; whereas the »ide b is become more oblitpie, 

 and of course its re^stance is much diminished, hi the 

 instance here represented, the angle of the |xirachute itself 

 is 144% and it is supposed to heel 18", the comparative re-- 

 distance of the side a to the side b, will b'e as the s<{uare of 

 the line a, as radius, to the square of the sine of the aii^le 

 of b with the current; which, being 54 degrees, gives the 

 resistances nearly m the ratio of 1 to 0-6"7; and this will be 

 reduced to only 0*544» when estimated in a direction per- 

 pendicular to the horizon. Hence, so far as this form of 

 the sail or plane is regarded, it operates directly in opposi- 

 tion to the principle of stability; for the side tliat is required 

 to fall resists much more in its new position, and that whidiy 

 is required to rise resists much less; therefore complete in- 

 verf.ion would be the consequence, if it were not for th» 

 weio'ht being suspended so very much below the sui-face, 

 which, counteracting this tendency, cQ.iverti* the effort into 

 a violent oscillation. 



and as it ought On the contrary, let the surface be applied in tli^ invertecl 



tw be made, position, as represented at C D, Fig. 2, and suppose it tp 

 be heeled to the same angle as before, represented by the 

 dotted lines c d. ' Here the exact reverse of the former iu- 

 •stance takes place; for that side, which is required to rise, 

 has gained resistance by its new position, and that which is 

 ..required to {jinjc bu,s lost it; i»o that as much power operates 



t» 



