254 PHILOSOPHICAL TRANSACTIONS. [aNNO 1751. 



injection-cock, by acting on the arm o m in such a manner, that when the 

 steam cock, is opened, the injection is shut, and vice versa, x is a bell of advice 

 which, moving along with the balance, continues to ring as long as the engine 

 is at work, v is a cock, which serves to discharge the air from the receiver, and 

 is opened by hand when necessary, w is a weight sufficient to raise the balance 

 to a perpendicular position, when it is inclined to the right, and also to overcome 

 the friction of the float, axis, pulleys, chain, &c. 



To put the engine in motion, press down the arm a b, which will bring the 

 balance over to the right side, and in its motion will open the steam-cock, and 

 shut the injection ; set open the cock at v, that the air may be discharged by 

 the entrance of the steam into the receiver. This done, shut that cock, and let 

 go the arm ; the weight w will bring over the balance to the left, and in its mo- 

 tion shut the steam-cock, and open the injection ; this presently condensing the 

 steam into water, in a great measure leaves a vacuum in the receiver. Things 

 remain in this situation, till the pressure of the atmosphere has caused the water 

 to mount through the suction-pipe into the receiver, where, as its surface rises, 

 it causes the float to ascend ; and, depressing the arm a b, raises the balance 

 till it has passed the perpendicular ; and in its descent, which is done by its own 

 gravity, the roller k lays hold of the arm r s, again opens the steam-cock, and 

 shuts the injection. The receiver being now almost filled with water, the ba- 

 lance cannot return, till the surface of the water in it subsides, and suffers the 

 float to descend. This is performed by the elasticity of the steam ; which, at the 

 same time that it fills the receiver, drives out the water through the forcing-pipe; 

 and when the surface is descended so low, as to suffer the weight w to bring the 

 balance beyond the perpendicular towards the lefl, it then falls of its own accord, 

 and in falling the roller i lays hold of the arm r s, shuts the steam-cock, and 

 opens the injection, as before. 



When the engine is to be stopped, observe when the balance lies to the right, 

 to turn round the arm o m of the injection-cock, so that the tail of the steam- 

 cock may miss it in the next motion ; so that at the same time that the receiver 

 is filled with steam, and the steam-cock shut, the injection not being opened, 

 the motion will stop for want of it. 



LXXIIL Concerning the Shells of Crabs. By Dr. Parsons, F. R. S. p. 439. 

 Dr. P. had no doubt of the animal's casting his shell at certain seasons ; he 

 only wanted to be satisfied, that the old exuviae were those of the soft crab ; 

 which the mutilated claw assured of, however difficult it might be to conceive 

 the manner of the animal's quitting it. The manner of his acquiring a new limb 

 is in nowise different from that of his obtaining a succeeding new shell ; which is 

 from a latent organization of the part ready for being indurated in due time. 



