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3892 JAMES WATT. 
which is open at the top, in proportion as it expands 
itself over their entire surface. This cooling is not com- 
pensated during the whole ascension of the piston, with- 
out the expense of a certain quantity of steam. But 
there is no loss of this sort in the engines modified by 
Watt. The atmospheric action is totally eliminated by 
the following means :— 
The top of the cylinder is closed by a metal cover, 
only pierced in the centre by a hole furnished with 
greased tow stuffed in hard, but through which the rod of 
the piston has free motion, though without allowing free 
passage either to air or steam. The piston thus divides 
the capacity of the cylinder into two distinct and well- 
closed areas. When it has to descend, the steam from 
the caldron reaches freely the upper area through a tube 
conveniently placed, and pushes it from top to bottom as 
the atmosphere did in Newcomen’s engine. There is no 
obstacle to this motion, because whilst it is going on, only 
the base of the cylinder is in communication with the 
condenser, wherein all the steam from that lower area 
resumes its fluid state. As soon as the piston has quite 
reached the bottom, the mere turning of a tap suffices to 
bring the two areas of the cylinder, situated above and 
below the piston, into communication with each other, 
so that both shall be filled with steam of the same degree 
of elasticity, and the piston being thus equally acted upon, 
upwards and downwards, ascends again to the top of the 
cylinder, as in Newcomen’s atmospheric engine, merely | 
by the action of a slight counterpoise. 
Pursuing his researches on the means of economizing 
steam, Watt also reduced the result of the refrigeration 
of the external surface of the cylinder containing the pis- 
ton, almost to nothing. With this view he enclosed the 
