JAMES WATT. 39 



would have occasioned material inconvenience. The air alone 

 besides, which in the old engine followed the piston in its 

 descent, acted with considerable effect in cooling the lower 

 part of the cylinder. His attempts to overcome this difficulty, 

 while they succeeded in that object, conducted Watt also to 

 another improvement, which effected the complete removal of 

 what we have called the second radical imperfection of New- 

 comen's engine — namely, its non-employment, for a moving 

 power, of the expansive force of the steam. The effectual way, 

 it occurred to him, of preventing any air from escaping into the 

 part of the cylinder below the piston, would be to dispense v/ith 

 the use of that element above the piston, and to substitute there 

 likewise the same contrivance as below, of alternate steam and 

 a vacuum. This was, of course, to be accomplished by merely 

 opening communications from the upper part of the cylinder to 

 the boiler on the one hand, and the condenser on the other, 

 and forming it at the same time into an air-tight chamber, by 

 means of a cover, with only a hole in it to admit the rod or 

 shank of the piston, which might, besides, without impeding its 

 freedom of action, be padded with hemp, the more completely 

 to exclude the air. It was so contrived accordingly, by a proper 

 arrangement of the cocks and the machinery connected with 

 them, that, while there was a vacuum in one end of the cylinder, 

 there should be an admission of steam into the other ; and the 

 steam so admitted now served, not only, by its susceptibility of 

 sudden condensation, to create the vacuum, but also, by its 

 expansive force, to impel the piston. Steam, in fact, was now 

 restored to be, what it had been in the early attempts to use it 

 as a mechanical agent, the moving power of the engine ; but its 

 efficiency in this capacity was for the first time both taken full 

 advantage ::)i, by means of contrivances properly arranged for 



