218 HYDRAULICS. 



cooled, he inclosed the top of the cylinder by 

 a metal plate having a hole in it, through which 

 the piston rod worked in a collar of leathers ; and 

 instead of employing the pressure of the atmos- 

 phere to force down the piston, he introduced the 

 steam above the piston, when the vacuum is formed 

 underneath, and employed it to produce this effect: 

 thus making the direct pressure of the steam the 

 moving power, as in the original construction of 

 the engine. 



The last great improvement made by Mr. Watt 

 was his giving an impulse to the piston by the 

 steam both in descending and ascending, instead 

 of being impelled, as in the old engine, during the 

 descent of the piston only. 



We shall now proceed to describe one of Watt's 

 engines on the latest construction. 



A is the boiler, which is generally of an oblong 

 form ; and the flame, after striking on its concave 

 bottom, circulates round the sides, and sometimes 

 returns in a pipe through the body of the water, 

 before it is suffered to go up into the chimney. 

 In his engines, there are commonly two of these 

 boilers, so that one of them may work while the 

 other is repairing. B (Plate 11. %. 1.), is the 

 steam-pipe which conveys the steam to the cylin- 

 der C, which is cased, and closed at top by a plate, 

 having a collar of leathers, through which the 

 piston-rod D works, a and c are the steam-valves, 

 through which the steam enters into the cylinder : 

 it is admitted through a, when it is to press the 

 piston downwards, and through c when it presses 

 it upwards, b and d are the eduction-valves, 

 through which the steam passes from the cylinder 

 into the condenser e, which is a separate vessel, 

 placed in a cistern of cold water, and which has a 

 jet of cold water continually playing up in the in- 



