424 



THE STEAM-ENGINE. 



If the atmosphere of the engine-house could be kept heated to the tempera- 

 ture of boiling water, this inconvenience would be removed. The piston would 

 then be pressed down by air as hot as the steam to be subsequently introduced 

 into it. On further consideration, however, it occurred to Watt that it would 

 be still more advantageous if the cylinder itself could be worked in an at- 

 mosphere of steam, having only the same pressure as the atmosphere. Such 

 steam would press the piston down as effectually as the air would ; and it would 

 have the further advantage over air, that if any portion of it leaked through be- 

 tween the piston and cylinder, it would be condensed, which could not be the 

 the case with atmospheric air. He therefore determined on surrounding the 

 cylinder by an external casing, the space between which and the cylinder he 

 proposed to be filled with steam supplied from the boiler. The cylinder would 

 thus be enclosed in an atmosphere of its own, independent of the external air, 

 and the vessel so enclosing it would only require to be a little larger than the 

 cylinder, and to have a close cover at the top, the centre of which might be 

 perforated with a hole to admit the rod of the piston to pass through, the rod 

 being made smooth, and so fitted to the perforation that no steam should escape 

 between them. This method would be attended also with the advantage of 

 keeping the cylinder and piston always heated, not only inside but outside ; 

 and Watt saw that it would be further advantageous to employ the pressure of 

 steam to drive the piston in its descent instead of the atmosphere, as its inten- 

 sity or force would be much more manageable ; for, by increasing or diminish- 

 ing the heat of the steam in which the cylinder was enclosed, its pressure might 

 be regulated at pleasure, and might be made to urge the piston with any force 

 that might be required. The power of the engine would therefore be completely 

 under control, and independent of all variations in the pressure of the atmosphere. 

 This was a step which totally changed the character of the machine, and 

 which rendered it a steam-engine instead of an atmospheric engine. Not 

 only was the vacuum below the piston now produced by the property of steam, 

 in virtue of which it is reconverted into water by cold ; but the pressure which 

 urged the piston into this vacuum was due to the elasticity of steam. 



The external cylinder, within which the working cylinder was enclosed, was 

 called the jacket, and is still very generally used. 



The first experiment in which Watt attempted to realize, on a small scale, 

 his conceptions, was made in the following manner. The cylinder of the engine 

 was represented by a brass syringe, A B (fig. 7), an inch and a third in diameter, 

 and ten inches in length, to which a top and a bottom of tin plate was fitted. 

 Steam was conveyed by a pipe, S, from a small boiler into the lower end of 

 this syringe, a communication being made with the upper end of the syringe 

 by a branch pipe, D. For the greater convenience of the experiment, it was 

 found desirable to invert the position of the cylinder, so that the steam should 

 press the piston P upward instead of downward. The piston-rod R therefore 

 was presented downward. An eduction pipe, E, was also inserted in the top 

 of the cylinder, which was carried to the condenser. The piston-rod was 

 made hollow, or rather a hole was drilled longitudinally through it, and a valve 

 was fitted at its lower end, to carry off the water produced by the steam, which 

 would be condensed in the cylinder in the commencement of the process. 

 The condenser used in this experiment operated without injection, the steam 

 being condensed by the contact of cold surfaces. It consisted of two thin 

 pipes, F G, of tin, ten or twelve inches in length, and the sixth of an inch in 

 diameter, standing beside each other perpendicularly, and communicating at 

 the top with the eduction pipe, which was provided with a valve opening up- 

 ward. At the bottom these two pipes communicated with another tube, I, of 

 about an inch in diameter, by a horizontal pipe, having in it a valve, M, open- 



