STEAM. 



steam, because the immediate agent in the former case is the 

 elastic force of the steam itself, while the agent in the latter case 

 is the atmospheric pressure, to which effect is given by the vacuum 

 produced by the condensation of steam. 



16. The three sources of mechanical power which have been 

 explained, have been used sometimes separately and sometimes 

 together in different forms of steam engine. 



In the class of engines commonly called high-pressure engines, 

 the direct power alone is used. In a class of engines, now out of 

 use, called atmospheric engines, the indirect power alone was 

 used. In the engines most generally used in the arts and manu- 

 factures, known as low pressure or condensing engines, both 

 powers are used. 



To obtain the mechanical effect of the vacuum produced by the 

 condensation of steam, it is not necessary that the atmospheric 

 pressure should be used. If we suppose that while the vacuum is 

 produced below the piston P, steam having a pressure equal to 

 that of the atmosphere be admitted to the upper side of it, the 

 piston will be urged downwards into the vacuum with the same 

 force exactly as if the atmosphere acted upon it. 



And, in effect, this is the method by which the indirect force of 

 steam is rendered effective in all engines as at present constructed, 

 the piston being in no case exposed to the atmosphere. 



17. In the preceding illustration of the power of steam, we 

 have supposed the piston P to have the area of a square 

 inch, and to be raised continuously to the height of 140 feet. But 

 it is evident that such conditions are neither necessary nor prac- 

 ticable. If the piston had an area of ten square inches the same 

 amount of evaporation would raise it to the tenth part of the 

 height ; but the force with which it would be raised, being 

 at the same time increased in a tenfold proportion, the me- 

 chanical effect would be the same, for it is evident that whether 

 15 Ib. be raised 140 feet, or 10 times 15 Ib. be raised the 

 10th part of 140 feet, the same mechanical effect would be 

 produced. 



The piston acted upon by the steam, instead of being continu- 

 ously driven in one direction, may be alternately elevated and 

 depressed, and still tho same amount of power will be developed. 

 Thus the evaporation may be continued until the piston has been 

 raised 10 foot. The steam which raised it may then be con- 

 densed, and the piston having descended to the bottom of the 

 tube, it may again be raised 10 feet by evaporation as before, and 

 this may be continued indefinitely. In this way, by means of a 

 short tube or cylinder, the mechanical effect attending the evapo- 

 ration of any quantity of water may be obtained, and this, in 

 201 



