18 THE HISTORY OF [SECT. i. 



to open and shut the cocks. This defect appears to have been first removed by 

 Gensanne, a Frenchman, who contrived a self-acting apparatus for the purpose in 

 1744 ; and afterwards De Moura, a Portuguese, sent a model of another method 

 to the Royal Society, which is ably described by Smeaton, in the Transactions for 

 1751. 1 His general description is sufficient for the purpose of showing how the 

 action is obtained. The engine consists of a receiver with a steam and an injec- 

 tion cock. It has a suction and a forcing pipe, each furnished with a valve, and 

 a boiler, which may be of the common globular shape. Having nothing particular 

 in its construction, a description of it will not be necessary : also the rest of the 

 parts already mentioned being essential to every machine of this kind, a further 

 account of them may be dispensed with. What is peculiar to this engine is a float 

 within the receiver, composed of a light ball of copper, which is not loose in it, but 

 fastened to the end of an arm made to rise and fall by the float, while the other 

 end of the arm is fastened to an axis ; and, consequently, as the float moves up 

 and down, the axis is turned round one way or the other. The axis is made 

 conical, and passes through a conical socket, which last is fixed to the side of the 

 receiver. On one of the ends of the axis, which projects beyond the socket, is 

 fitted a second arm, which is also moved backwards and forwards by the axis as 

 the float rises or falls. By these means, the rising or falling of the surface of the 

 water within the receiver communicates a corresponding motion to the outside, 

 in order to give the proper motions to the rest of the apparatus which regulates 

 the opening and shutting of the steam and injection cocks, and serves the same 

 purpose as the plug frames, &c. in Newcomen's engine. 



1751. FRANCIS BLAKE, F.R.S. 



19. A paper on the best proportions for steam engine cylinders, by Mr. Francis 

 Blake, was published in 1751 ; 2 which merits attention both as one of the first 

 steps in theoretical inquiry respecting the proportions of engines, and on account 

 of the result he obtained. It is evident, he remarks, from the principles of 

 mechanics, that the contents of the cylinder remaining the same, the quantity of 

 water discharged at each lift will in all cases be equal ; and this equality is obtained 

 by only adjusting the distance of the centre of the piston from the fulcrum of the 

 beam. It will be granted also, that the excess of the column of atmosphere above 

 that of water, is equivalent to a weight on the piston, driving it to a depth of 

 about five feet within the cylinder with an accelerated motion, till friction and 

 resistance from the uncondensed steam which remains in the cylinder even after 

 the injection, and which is increased in elasticity while its bounds are dimi- 



1 Phil. Trans, vol. xlvii. p. 436. or Abridg. vol. x. p. 252. 



2 Phil. Trans, vol. xlvii. p. 197. or Abridg. vol. x. p. 187. 



