90 MECHANICS. 



High Pressure. 



In a vacuum water boils at about 70; under 

 common atmospheric pressure at 212, when the 

 steam ascends from the bottom and carries off the 

 latent heat, preventing any further elevation of tem- 

 perature. 



Add one artificial atmosphere, by loading the 

 escape-valve (the surface of which is equal to a 

 square inch) with 14 pounds, and the water will 

 acquire 250 of heat with a very small addition of fuel, 

 and the pressure on the square inch will be doubled, 

 or 28 pounds. Loaded with four atmospheres or 42 

 pounds, the temperature will be raised to 280, with 

 a pressure equal to 56 pounds on the square inch, 

 and so on in proportion to the load. 



The principle of high pressure steam engines de- 

 pends on the power of steam to expand itself 5, 10, 

 20, 30, 40, &c, times, beyond its original bulk, by the 

 addition of a given portion of heat, which is effected 

 by increasing the pressure. 



In high pressure engines the steam is not con- 

 densed, but after having acted on the piston is allowed 

 to blow off into the air, whereas in low pressure en- 

 gines it passes into a separate vessel, where it is con- 

 densed ; on which account, and for other reasons, low 

 pressure engines do not suit a rail-road. High pres- 

 sure engines occupy less room, require rather less 

 fuel than low pressure, and their power can be 

 increased on emergencies by merely increasing the 

 fire ; but the risk of damage from explosion is con- 

 siderable. Their principal purpose is to save water, 

 but this is always abundant in navigation. Low 

 pressure engines cease to be such whenever the fire 

 is strongly urged and the valve overloaded. 



Rarefaction. 



The rarefaction obtained by an air pump, in 1783, 

 went to about the i^otf^ P art f an atmosphere ; 



