168 FERGUSON'S LECTURES. 



LECT. vvater to run from the cistern into the interior of the condenser 

 in such a direction as to meet the steam, by which the condeu- 

 w sation was not only rendered more rapid but more perfect : this 

 is called the injection cock. 



In this state of the engine Mr. Watt found that it consumed 

 but little more than half the fuel required by all the other con- 

 structions that had preceded it. But still he was not satisfied 

 with the perfection of his machine; for the cylinder had an 

 open top, and required water to lie upon the top of the piston to 

 keep it air-tight ; consequently whenever the piston descended, 

 the whole interior surface of the cylinder was not only exposed 

 to the open air, but was coated with a thin layer of cold water, 

 which, by the previously acquired heat of the cylinder, was imme- 

 diately converted into vapour, thus cooling it to a great degree, 

 so that a material waste of heat still existed, which none of the 

 before-mentioned improvements could remove. Mr. W^att 

 therefore determined on trying the effect of a cylinder with a 

 close top, or one which should effectually shut out the external 

 air, by causing the rod of its piston to slide through a stuffing 

 box, and instead of admitting the atmosphere to depress the 

 piston, he made use of steam for that purpose, as well as to 

 produce the vacuum. This expedient he was persuaded he 

 could resort to, because the air's pressure could at no time exceed 

 about 15 pounds on the square inch, and he knew that the elastic 

 force of steam had been used to a much greater extent in Sa- 

 very's engine, and could be productive of no danger, while it 

 was kept within the moderate limits he required ; for since the 

 steam in his new machine had to act against a vacuum under 

 the piston, instead of against the resistance of the open air, so 

 at the heat of 212 degrees it would be fully equivalent to the 

 pressure of the atmosphere ; and by using it but a very little hot- 

 ter it would even be superior to it. His only doubt therefore 

 was, whether this additional expenditure of steam might not oc- 

 casion a greater consumption of fuel than the waste that occurred 

 through the cooling of the cylinder, but the trial of the experi- 

 ment set this point at rest, and proved most clearly the value of 

 his suggestion, and in his future engines he carried the improve- 

 ment so much further, both in the single and double acting en- 

 gines, as they were afterwards called, as to require no more steam 

 for this new mode of operating, than had formerly been required 

 to produce the vacuum only. The machine was now for the first 

 time actually converted into a steam, instead of an atmospheric 

 engine, for atmospheric pressure was entirely excluded, and it 



