221< HYDRAULICS. 



up again, its valve now remaining shut by its 

 weight. On arriving at the top, it presses up the 

 steam valve, which admits the steam from the 

 boiler to force it down as before. 



/and m are two cranks, upon whose axes are two 

 equal wheels working in each other, for the pur- 

 pose of converting the perpendicular motion of 

 the piston-rod into a rotatory motion, for working 

 the machinery attached to it. 



But the most valuable part of this engine is in 

 the construction of the piston, which Mr. Cart- 

 wright made wholly of metal, and so as, by means 

 of strings, to fit the cylinder very exactly. This 

 not only saves the expence and trouble of packing, 

 which must frequently be renewed in all other 

 engines, but alsojsaves a great deal of steam, on 

 account of the more accurate fitting of the piston. 



As it is evident from its construction that the 

 whole of the steam is brought back again into the 

 boiler, it affords the means of employing alkohol 

 instead of water, and thus saving a great deal of 

 fuel. 



This machine seems to be peculiarly applicable 

 to purposes requiring only a small power, as it is 

 not expensive, and occupies little room. 



The draining of deep mines requiring a very 

 great power, steam heated to above the usual tem- 

 perature of 212 , is sometimes employed in steam 

 engines, and these are called high pressure engines. 

 Several of these have been executed in Cornwall 

 by Woolf and Trevethick. They are the most 

 powerful of this class of engines which have been 

 constructed, but they require extraordinary strength 

 in the boilers, and great care with the safety 

 valves, a neglect of which has occasioned dreadful 

 explosions. 



