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 lbs. 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 lbs. 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 lbs. 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