g^ ON AFRfAL NAVIGATION. 



circumstance gravity being btlow the point of siispeiisior,, is aided by a 

 ai'.hng thissta- ,.eit)arkable ciicuiiisttuict:, tluit experiuieut aloiie could point 

 out. lu very acute angles with th^ euneut it apijears, that 

 the centre of resistance iu the sail docs not .coiucide with 

 the centre of its surface, bitl is contiderably in fsont of it. 

 An the obhqnity of the current decrea^ies, these cer.tres ap- 

 proach, and coincide when the current becomes perpendi- 

 cular to the t;ail. IJeuce any heel of the machine backward 

 or forward removes the ceuire of support behind or before 

 the point of suspension ; and operatefi to restore the original 

 position, by a power, equal to the wlsole weight of the ma- 

 chine, acting upon a lever equal iu length to the distance 

 the centre has removed. 

 A rudder ne- To render the machine perfectly steady, and likewise to 

 ^''*V7.,'''i''^'/'^ enable it to ascend and descend in its path, it becomes ne- 

 btrd. cessary to add a rudder in a similar position to the tail in 



birds. Let F G be the section of such a surface, parallel 

 to the current; and let it be capable of moving up and. 

 down upon G, as a centre, and of being tixed in any posi- 

 tion. The powers of the machine beitig jireviously balanced, 

 if the least pressure be exerted by the current, either upon 

 the upper or under surface of the rudder, according to il;e 

 will of the aeronaut, it will cause the machine ro rise or 

 fall in its path, so long as the projectile or propelling force 

 is continued with sufficient energy. From a variety of ex- 

 periments upon this subject 1 (ind, that, when the machine 

 is going forward with a feU[)erabundahi velocity, or that 

 which would induce it to ri^e in iis path, a very sUady iio- 

 rizonta! course is etlected by a con^pideiaijie dep.ei-sion of 

 the rudder, which has tlie adva.itage of making use of this 

 portion of sail in aiding the support of the v.eight. When 

 the velocity is becoming less, as in the act of aligljtiug, then 

 the rudder must gsadually recede fiom t!iis position, and 

 even become elevated, ior the p\irpose of preventing the 

 machine from sinking too much in fiont, owitig to the com- 

 bined effect of the want of projectile force sufficient to sus- 

 tain the centre of gravity in its usual position, and of the 

 centre of su{)port approaching the centre of the sail. 

 K.irOirtr M-^e of '^ '''^ elevation and depression of the ip.achu.e are not the 

 ike rudder. oi,]y purposes, for which tlie rudder is desigacd. This ap- 



