3oo HISTORY OF SCIENCE. 



great defect of this machine was, that after the piston had descended 

 and the water had been forced out of the cylinder by the exit-pipe 0, 

 the cylinder itself had necessarily been cooled, partly by the injected 

 water, and partly by its interior becoming exposed to the contact of 

 the atmosphere ; for the cylinder was quite open at the end. Hence 

 on the readmission of the steam a large quantity was condensed by 

 the cold cylinder, until the latter had acquired the temperature of 212. 

 When Watt began the study of Newcomen's engine, with a view of 

 employing steam in a more effective manner, he proceeded, as already 

 stated, in a truly scientific method, by first ascertaining all the pro- 

 perties of steam. Having by a long series of experiments learnt the 

 qualities of the agent with which he had to work, he considered the 

 mode of its application ; and he found that of the total quantity of 

 steam required to work Newcomen's engine, three-fourths was ex- 

 pended in re-heating the cylinder. Therefore if the cylinder, instead 

 of being cooled at each stroke, could be kept constantly hot, a fourth 

 part of the steam, and consequently a fourth part of the fuel hitherto 

 used, would suffice. Besides all this, the heated cylinder always con- 

 verted some of the injected cold water into steam, which diminished 

 the force effecting the descent of the piston by about one-fourth, even 

 in the best of Newcomen's engines. 



Watt thought over the means of overcoming these defects, and a 

 .simple but beautifully effective plan presented itself to his mind. He 

 proposed to connect another vessel with the cylinder by means of an 

 open pipe, and when the cylinder and the vessel were full of steam, 

 to inject cold water into the vessel, when, a vacuum being produced 

 there, more steam would rush in from the cylinder, and would be con- 

 densed in its turn, and so on so long as any remained. Thus the re- 

 quired vacuum would be produced without a single drop of cold water 

 coming into contact with the cylinder. The invention of the sepa- 

 rate Condenser was the grand improvement which Watt effected in the 

 steam-engine ; but there was another of first-rate importance, and that 

 consisted in preventing all access of the atmosphere to the interior of 

 the cylinder. The cold air, if allowed to urge the piston, would of 

 course cool the interior walls of the cylinder, and loss of much steam 

 would result. Watt substituted the tension of the steam from the 

 boiler for the atmospheric pressure, and there was always steam on 

 one side of the piston, a vacuum on the other. The top of the cylinder 

 was no longer open as in Newcomen's engine, but was closed by a 

 steam-tight cover, a hole in the centre of which admitted free passage 

 for the piston-rod, while the passage of air or steam was prevented by 

 packing very tightly round the opening with greased tow. The general 

 arrangement of the double-action engine as perfected by Watt is shown 

 in the diagram, Fig. 147, where A is the cylinder, i the condenser, j a 

 pump worked by the engine for drawing the injection water and con- 

 densation water from the condenser into the hot 7c>e//,j\ where it is 



