222 HYDRAULICS. 



statics, already explained, a certain part of the 

 weight of the stone will be supported by the water, 

 so long as it continues immersed in it; but if a 

 part of the water evaporate by boiling, a pro- 

 portional part of the stone will be above the water, 

 consequently the stone will bear more upon the 

 lever, and raise the weight at the other end; but 

 in raising that weight, it also opens the valve in 

 the small cistern, and admits water until it stand 

 at the same height in the boiler as before, and 

 then the valve and the stone being again in 

 equilibrio, the valve remains shut until a new 

 quantity is evaporated. By this means the supply 

 of water is very gradual, however, and not by fits 

 and starts, as here described for the sake of 

 illustration. 



It is found by experience to be a much better 

 method than a ball-cock; and the regular sup- 

 plying of the boiler with water is of the first im- 

 portance. As a check upon this, and to know 

 perfectly the height of the water in the boiler, 

 there are two cocks, g and h> one of which reaches 

 nearly to the surface of the water when at its 

 proper height, and the other enters a little below 

 the surface. 



It is evident, that if the water be at the just 

 height, and you open g, that steam will issue; and 

 if h be opened, water will be driven out by the 

 pressure of the steam. But if water come out 

 from g y then the water must be too high in the 

 boiler ; and if steam issue from h, then the water 

 is too low. By this means, it is easy to know at all 

 times the exact height of the water in the boiler. 



i is a safety-valve, to prevent the bursting of the 

 boiler by the steam growing too strong; k is the 

 pipe which conveys the steam to the engine. 



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