OR, PLAIN 



The boiler is fitted with gauge 

 cocks, which serve to indicate the 

 depth of water in the boiler ; on 

 the side opposite to the gauge 

 cocks is a safety-valve, 5, that is, 



a valve which opens with less 

 force than is necessary to burst 

 the boiler, so that when the steam 

 acquires a force approaching the 

 estimated strength of the boiler, 

 the valve opens, and reduces the 

 pressure, by letting off a portion 

 of the steam. The piston, 6, is 

 fitted so tightly that it will allow 

 neither air nor steam to pass by 

 its edges into or from the cylinder, 

 yet will itself move freely up and 

 down. There is a pipe, 7, 8, 

 leading into the cylinder, 2, con- 

 veying cold water from a cistern, 

 9. This pipe, called the injection 

 pipe, is furnished with a stop-cock, 

 situated at 8, not shown in the 

 engraving, by which the water 

 may be turned off or on. The | 



TEACHING. 101 



water cistern, 9, is supplied by a 

 pump worked by the beam,\Z, 

 which raises the water through 

 the pipe, 10. 



The mode in which this engine 

 works is thus : the valve, 3, being 

 opened, steam rushes into the 

 cistern, 2 ; this, together with 

 the action of weights, 16, at the 

 opposite end of the beam, raises 

 the piston, 6, to the top. A 

 valve at 11, opens and lets out 

 any air that may be in the cylin- 

 der, with some of the steam, 

 until the cylinder is completely 

 filled with pure steam ; the valve, 

 3, is now closed, and a jet of 

 water forced into the cylinder, 2, 

 through the injection pipe, 8 ; 

 this suddenly condenses the steam, 

 and then the air, pressing upon 

 the upper surface of the piston, 

 forces it down to its original posi- 



e 



379. 



tion. This upward and down- 

 ward movement of the piston, 6, 

 is communicated to the beam, 13, 

 by a chain and rod, 15, and the 

 rising and falling alternately of 

 the opposite ends of this beam, 



