SECT, i.] THE STEAM ENGINE. 11 



the forcing pump R. From this descends the injection pipe M, which enters the 

 cylinder through its bottom, and terminates in one or more small holes at N. This 

 pipe has at O a cock, called the injection cock, fitted with a handle. At the 

 opposite side of the cylinder, a little above its bottom, there is a lateral pipe, 

 turning upwards at the extremity, and provided with a valve at V, called the 

 snifting valve, which has a little dish round it to hold water for keeping it 

 air-tight. 



There proceeds also from the bottom of the cylinder a pipe Q, of which the lower 

 end is turned upwards, and is covered with a valve v : this part is immersed in a 

 cistern of water called the hot well, and the pipe itself is called the eduction pipe. 

 To regulate the strength of the steam in the boiler, it is furnished with a safety 

 valve, constructed and used in the same manner as that of Savery's engine, but not 

 loaded with more than one or two pounds on the square inch. 



The mode of operation remains to be described. Let the piston be pulled down 

 to the bottom of the steam cylinder, and shut the regulator or steam valve p. 

 Then the piston will be kept at the bottom by the pressure of the atmosphere. 

 Apply the fire to the boiler till the steam escapes from the safety valve, and then, 

 on opening the steam regulator, the piston will rise by the joint effect of the 

 strength of the steam, and action of the excess of weight on the other end of the 

 beam. When it arrives at the top of the cylinder, close the regulator p, and, by 

 turning the injection cock O, admit a jet of cold water, which condenses the steam 

 in the cylinder, forming a partial vacuum, and the piston descends by the pressure 

 of the atmosphere, raising water by the pump rod H from the mine. The air 

 which the steam and the injection water contain, is impelled out of the snifting 

 valve V, by the force of descent, and the injection water flows out at the eduction 

 pipe Q ; and by repetition of the operations of alternately admitting steam and 

 injecting water, the work of raising water is effected. 



These operations were done by hand, till a boy, named Humphrey Potter, con- 

 trived to attach strings and catches to the working beam, for opening and shutting 

 them while he was at play ; 1 after which, more permanent appendages were added 

 to answer the purpose, and the engine became a step nearer to a self-regulating 

 machine. 



The engine in this simple and efficient state was termed the Atmospheric 

 Engine. It was brought to this degree of perfection about 1712; and such 

 engines were erected in various places. 



The novelty of this engine is chiefly in its mechanism ; but as this mechanism 

 produces all the difference between an efficient and an inefficient engine, I am 



1 Desaguliers' Experimental Philosophy, ii. 533. 



