THE STEAM ENGINE. 



turning the wheel w. If F be plunged deeper in the water, A 

 more than balance it, and will pull it up, so that the only position 

 in which F and A will balance each 

 other is, when F is half immersed. 

 The wheel w is so adjusted, that 

 when two pins placed on its rim. 

 are in the horizontal position, the 

 water is at its proper level. Con- 

 sequently it follows, that if the water 

 rise above this level, the weight E 

 is lifted and A falls, so that the pins 

 come into another position, and if 

 it fall lower, F falls and A rises, so 

 that the pins assume a differ en'; 

 position. Thus, in general, tho 

 position of the pins becomes 



indication of the quantity of water in the boiler. 



Another method is to place a glass tube (fig. 3), with one end 

 entering the boiler above the proper level, and the other end T' 

 entering it below the proper level. It must be evident that the 

 water in the tube will always stand at the same level as the water 

 in the boiler, since the lower part has a free communication with 

 that water, while the surface is submitted to the pressure of 



the same steam as the water in the boiler. 



This and the last-mentioned gauge have 

 r the advantage of addressing the eye of the 



engineer at once, without any adjustment ; 



whereas the gauge- cocks must be both 



opened, whenever the depth is to be 



ascertained. 



These gauges, however, require the 



constant attention of the engine-man; 



and it becomes desirable either to find some more effectual 

 means of awakening that attention, or to render the supply 

 of the boiler independent of any attention. In order to enforce 

 the attention of the engineman to replenish the boiler when 

 partially exhausted by evaporation, a tube was sometimes 

 inserted at the lowest level to which it was intended that the 

 water should be permitted to fall. This tube was conducted from 

 the boiler into the engine-house, where it terminated in a mouth- 

 piece or whistle, so that whenever the water fell below the level 

 at which this tube was inserted in the boiler, the steam would 

 rush through it, and issuing with great velocity at the mouth- 

 piece, would summon the engineer to his duty with a call that 

 would rouse him even from sleep. 

 C 



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