1)1 the Construction of Steam- Engines. 339 



measure for the larger. For example, if steam of forty 

 pounds the square inch is fixed on, then the smaller steam- 

 vessel should be at least one-fortieth part the contents of 

 the larger one. Each steam-vessel should be furnished 

 with a piston, and the smaller cylinder should have a com- 

 munication both at its top and bottom (top and bottom 

 being here employed merely as relative terms, for the cy- 

 linders may be worked in a horizontal, or any other re- 

 quired position, as well as vertical). The small cylinder, 

 I say, should have a communication both at its top and 

 bottom with the boiler which supplies the steam ; which 

 communications, by means of cocks or valves of any con- 

 struction adapted to the use, are to be alternately opened 

 and shut during the working of the engine. The top of 

 the small cylinder should have a communication with the 

 bottom of the larger cylinder, and the bottom of the smaller 

 one with the top of the larger, with proper means to open 

 and shut these alternately bv cocks, valves, or any other 

 well known contrivance. And both the top and bottom of 

 the larger cylinder or steam-vessel should, v>'hile the engine 

 is at work, communicate alternately with a condensing-ves- 

 sel, into which a jet of water is admitted to hasten the con- 

 densation ; or the condensing-vessel may be cooled by any 

 other means calculated to produce that efiect. Things 

 being thus arranged, when the engine is at work, steam 

 of high temperature is admitted from the boiler, to act by 

 its elastic force on one side of the smaller piston, while the 

 steam which had last moved it has a communication with 

 the larger steam-vessel or cylinder where it follows the 

 larger piston now moving towards that end of its cylinder 

 which is open to the condensing-vessel. Let both pistons 

 end their stroke at one time, and let us now 'suppose them 

 both at the top of their respective cylinders, ready to de- 

 scend ; then the steam of forty pounds the square inch en- 

 tering above the smaller piston will carry it downwards, 

 while the steam below it, instead of being allowed to escape 

 into the atmosphere, or applied to any other purpose, will 

 pass into the lanjer cylinder above its piston, which will 

 take its downward stroke at the same time that the piston 

 of the smaller cvlinder is doing the s;nne thing; and while 

 this goes on, the steam which last filled the larger cylinder 

 m the upward stroke of the engine will be passed into the 

 condenser, to be condensed during the downward stroke. 

 When the pistons in the smaller and larger cylinder have 

 thus been made to descend to the bottom of their respective 

 cylinders, then the steam from the boiler is to be shut oft' 

 Y a from 



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