SECTION V. 



OF THE CONSTRUCTION OF NONCONDENSING ENGINES. 



356. NONCONDENSING ENGINES, usually called high pressure engines, are moved 

 by steam generated under a considerable degree of pressure ; and it is the excess of 

 this pressure above the pressure of the atmosphere, which constitutes their power to 

 produce motion. From 30 to 40 Ibs. on a circular inch is the excess above 

 atmospheric pressure, commonly employed in this country. 



357. The working parts of the engine consist of a cylinder, having passages 

 provided with cocks or valves for steam to enter into it, either at the top or at the 

 bottom ; and also the means of letting out the steam to the atmosphere, either at 

 the top or bottom. The cylinder has an air-tight piston, to be moved from one 

 end to the other by the pressure of the steam, with a rod fixed to it, called the 

 piston rod, which slides through an air-tight box at the top of the cylinder, to 

 give motion to a crank or some other piece of machinery. 



358. Now, with steam in the boiler having a force of 30 Ibs. to the cir- 

 cular inch, if the piston be at the bottom of the cylinder, and the passage from 

 the boiler to the bottom, and that to the atmosphere at the top, be both open, and 

 the rest shut, the steam will exert a pressure of nearly 30 Ibs. on each inch of 

 the area of the piston, and cause it to ascend. A little before it arrives at the 

 top, the cocks must be shut, and the moment it has got to the top, the other two 

 cocks should be opened ; the steam from the boiler will then press the piston 

 downwards, and the steam before let in will flow out into the open air. Again, the 

 passages must be closed a little before the completion of the stroke, and in this 

 manner the operation may be continued. 



359. The close of the cocks before the termination of the stroke prevents either 

 concussion against the end of the cylinder, or strain on the crank shaft; and, when 



