406 



THE STEAM-ENGINE. 



Two gauge-cocks are represented at G G', the use of which is to determine 

 the depth of water in the boiler. One, G, has its lower aperture a little above 

 the proper depth ; and the other, G', a little below it. Cocks are attached to 

 the upper ends G G', which can be opened or closed at pleasure. The steam 

 collected in the top of the boiler pressing on the surface of the water, forces it 

 up in the tubes G G'. if their lower ends be immersed. Upon opening the 

 cocks G G', if water be forced from both, there is too much water in the boiler, 

 since the mouth of G is below its level. If steam issue from both, there is too 

 little water in the boiler, since the mouth of G' is above its level. But if steam 

 issue from G, and water from G', the water in the boiler is at its proper level. 

 This ingenious contrivance for determining the level of the water in the boiler 

 is the invention of Savery, and is used in many instances at the present day. 



The mouth of the pipe G should be at a level of a little less than one third 

 of the whole depth, and the mouth of G' at a level little lower than one third, 

 for it is requisite that about two thirds of the boiler should be kept filled with 

 water. The tube I forms a communication between the greater boiler D and 

 the lesser or feeding boiler E, descending nearly to the bottom of it. This 

 communication can be opened and closed at pleasure by the cock K. A gauge 

 pipe is inserted similar to G G', but extending nearly to the bottom. From 

 this boiler a tube, F, extends, which is continued to a cistern, C (fig. 3), and 

 a cock is placed at M, which, when opened, allows the water from the cistern 

 to flow into the feeding boiler E f and which is closed when that boiler is filled, 

 The manner in which this cistern is supplied will be described hereafter. 



Let us now suppose that the principal boiler is filled to the level between 

 the gauge-pipes, and that the subsidiary boiler is nearly full of water, the cock 

 K and the gauge-cocks G G' being all closed. The fire being lighted beneath 

 D, and the water boiled, steam is produced, and is transmitted through one or 

 other of the tubes T T', to the working apparatus. When evaporation has 

 reduced the water in D below the level of G, it will be necessary to replenish 

 the boiler D. This is effected thus : — A fire being lighted beneath the feed- 

 ing-boiler E, steam is produced in it above the surface of the water, which, 

 having no escape, presses on the surface so as to force it up in the pipe I. 

 The cock K being then opened, the boiling water is forced into the principal 

 boiler D, into which it is allowed to flow until water issues from the gauge- 

 cock G'. When this takes place, the cock K is closed, and the fire removed 

 from E until the great boiler again wants replenishing. When the feeding- 

 boiler E has been exhausted, it is replenished from the cistern C (fig. 3), 

 through the pipe F, by opening the cock M. 



We shall now describe the working apparatus in which the steam is used 

 as a moving power. 



