PERPETUAL MOTION, AND JOINTfiD PARALLEL RULE. 213 



equal to that in the clofed veflel A. Thefe two vefTels are 

 conneded by a long horizontal tube G. It is evident that any 

 change, either in the prefTure of the external air or the elafticity 

 bf the internal air, will caufe the fluid to run along the tube, 

 and add to the weight of A or of B according to circumftances. 

 The heavier veflel will preponderate ; but it will be prevented 

 from defcending too far by a flop or bearing to which it will 

 arrive. Any change in the inclination of G will move the at- 

 tached lever C D ; by means of which, one of the two hori- 

 zontal racks will be made to pulh round that ratchett wheel into 

 which its teeth fall, at the fame time that the other rack will be 

 drawn backwards upon its wheel. The oppofile adion will 

 drive forward the other wheel ; and as both thefe wheels are 

 fixed on the fame axis, the fyllem will be driven the fame 

 way by every change of denfity or weight in the air that takes 

 place. 



The other iiiHrument, Fig, 3, is a parallel rule which is Account of a 



franied without any Hiding work, and opens to a much greater "^^ ^^"^^ 



extent than ufual, without any fide deviation. I do not know 



the inventor; but it was communicated to me by a private 



hand. The jointed parts are reprefented in Fig. 2, and the 



dimenfions are exprefled by the fmall numerals. From the 



joint D to the joint E upon the bar, C E, the difiance, is = 



1 ; from the joint E to the joint A, = 2 ; from the joint A 



to the joint B, = 3 ; and from the joint B lo the joint D, = 



,,4. ; which is alfo equal to the part D C of the Bar E C. Now 



ft is found, and I muft leave the mathematical proof to your 



readers, that the point C moves by the opening from B (upon 



which it lies when (hut) in a firait line, very nearly at right 



angles with B A. From this property it is eafily feen, that 



two parallel rules, having the fyftem of four bars fixed to 



each, and conne6led as in the figure, will be moveable without 



fide deviation, and will preferve their paraHelifm in all 



, fitualions. 



I am Sir, 



Your obliged humble fervant, 



R. B. 



Fankiliar 



