410 



THE STEAM-ENGINE. 



ble elasticity would, notwithstanding the condensation, remain in the vessels 

 V V' and the pipe S, and would oppose the ascent of the water. In conse- 

 quence of this, the engine could never be placed, with practical advantage, at 

 a greater height than twenty-six feet above the level of the water to be raised. 



When the water is elevated to the engine, and the steam-vessels filled, if 

 steam be introduced above the water in V, it must first balance the atmospheric 

 pressure, before it can force the water through the valve B. Here, then, is a 

 mechanical pressure of fifteen pounds per square inch expended, without any 

 water being raised by it. If steam of twice that elastic force be used, it will 

 elevate a column in F of thirty-four feet in height ; and if steam of triple the 

 force be used, it will raise a column of sixty-eight feet high, which, added to 

 twenty-six feet raised by the atmosphere, gives a total lift of ninety-four feet. 



In effecting this, steam of a pressure equal to three times that of the atmo- 

 sphere acts on the inner surface of the vessels V V 7 . One third of this burst- 

 ing pressure is balanced by the pressure of the atmosphere on the external 

 surface of the vessels ; but an effective pressure of thirty pounds per square 

 inch still remains, tending to burst the vessels. It was found that the appa- 

 ratus could not be constructed to bear more than this with safety ; and, there- 

 fore, in practice, the lift of such an engin*e was limited to about ninety perpen- 

 dicular feet. In order to raise the water from the bottom of the mine by these 

 engines, therefore, it was necessary to place one at every ninety feet of the 

 depth ; so that the water raised by one through the first ninety feet should be 

 received in a reservoir, from which it was to be elevated the next ninety feet 

 by another, and so on. 



Beside this, it was found that sufficient strength could not be given to those 

 engines, if constructed upon a large scale. 



They were, therefore, necessarily very limited in their dimensions, and 

 were incapable of raising the water with sufficient speed. Hence arose a ne- 

 cessity for several engines at each level, which greatly increased the expense. 



These, however, were not the only defects of Savery's engines. The con- 

 sumption of fuel was enormous ; the proportion of heat wasted being much 

 more than what was used in either forcing up the water, or producing a vacu- 

 um. This will be very easily understood, by attending to the process of work- 

 ing the engine already described. 



When the steam is first introduced from the boiler into the steam-vessels 

 V V 7 , preparatory to the formation of a vacuum, it is necessary that it should 

 heat these vessels up to the temperature of the steam itself ; for until then the 

 steam will be condensed the moment it enters the vessel, by the cold surface. 

 All this heat, therefore, spent in raising the temperature of the steam-vessels 

 is wasted. Again, when the water has ascended and filled the vessels V V 7 , 

 and steam is introduced to force this water through B B 7 into F, it is immedi- 

 ately condensed by the cold surface in V V, and does not begin to act until a 

 quantity of hot water, formed by condensed steam, is collected on the surface 

 of the cold water which fills these vessels. Hence another source of the waste 

 of heat arises. 



When the steam begins to act upon the surface of the water in V V 7 , and to 

 force it down, the cold surface of the vessels is gradually exposed to the steam, 

 and must be heated while the steam continues its action ; and when the water 

 has been forced out of the vessel, the vessel itself has been heated to the tem- 

 perature of the steam which fills it, all which heat is dissipated by the subse- 

 quent process of condensation. It must thus be evident, that the steam used 

 in forcing up the water in F, and in producing a vacuum, bears a very small 

 proportion, indeed, to what is consumed in heating the apparatus after con- 

 densation. 



