OF STEAM ENGINES. 



169 



worked solely by the power of steam : in consequence of which 

 the cylinder could at all times be kept as hot as the steam which 

 had to enter it, and all the immense loss of fuel that had hither- 

 to attended the use of this valuable machine was completely 

 obviated, while its work was rendered regular and conti- 

 nuous. 



The double acting engine requires twice the quantity of steam 

 that would be necessary for a single engine of the same size : 

 but this is compensated for by its doing twice the quantity of 

 work. And after what has been said upon the single engine, 

 little need be added in explanation of the double one, for its 

 principles being the same, a mere change of mechanism is all 

 that is required to produce the machine. No compensation 

 weights are necessary to raise the steam piston, and such an 

 alteration must be made in the valves and steam pipes that a 

 vacuum may be produced by the condenser, alternately above 

 and below the piston, instead of below it only, as in the engines 

 before noticed ; the steam likewise must be admitted alternately 

 above and below the piston, at the same moment that the vacuum 

 is formed, but always on that side of the piston that is opposite 

 to the vacuum. 



In order to a due comprehension of the action of the double 

 engine, some of the contrivances that have been adopted to pro- 

 duce a proper distribution of the steam, and an alternate vacuum, 

 must next be noticed, and although the four-way cock, and 

 sliding regulator are not considered as the most perfect of these, 

 yet they are very frequently applied to small steam engines, and 

 their simplicity gives them a preference. Let YZ represent a 

 section of a steam engine cy- 

 linder as before, with its pis- 

 ton, and the piston rod moving 

 through a stuffing box, so as 

 to be quite air-tight, g is the 

 steam pipe for bringing steam 

 from a boiler, and this termi- 

 nates in a four-way cock 

 h i k, which, together with the 

 pipes m n and o, are drawn 

 much too large for the cylin- 

 der YZ, in order that their 

 parts may be distinctly seen : 

 p is the end of a lever or han- 

 dle for the purpose of turning 



