3-26 ilevitiv of Renivicic 



But we cannot fairly estimate the degree of strength necessary to 

 be maintained in the boiler, and the other parts of the apparatus, or 

 understand the various precautions necessary in order to insure 

 safety in the ifse of the steam engine, without understanding very 

 fully the nature of the moving force. Let us therefore review the 

 LEADING FACTS RESPKCTiNG STEAM, especially such as relate to the 

 subject before us. 



1. It will be recollected, that the great *and peculiar property, on 

 which the mechanical agencies of steam depend, is its power of ex- 

 erting at one moment a high degree of elastic force, and losing it in- 

 stantaneously the next moment. This force, acting on the bottom of 

 the piston, which moves in the main Cylinder, raises it, and fills the 

 space below it with steam. The steam is suddenly condensed, and 

 hence no obstacle is opposed to the descent of the piston, but it is 

 readily forced down again by steam acting from above. This alter- 

 nate motion of the piston, the rod of which is connected with the 

 working beam, is all that is required in order to communicate motion 

 to all parts of the engine. 



2. The elastic force of steam depends on the temperature at 

 which it is formed ; and the temperature necessary to its production 

 depends upon the pressure incumbent upon the water during its for- 

 mation. 



Water is capable of forming vapor at all temperatures whatsoever. 

 Its tendency to rise is, however, impeded by pressure, and thus it 

 does not boil in an open vessel, when the rising of steam is impeded 

 by the resistance of the atmosphere, until it reaches the temperature 

 of 212°. But with each diminution of pressure, the boiling tempera- 

 ture becomes lower, until, in the vacuum of an air pump, it boils at 

 90°, [70°?]. Hence, so soon as a portion of the steam is condensed, 

 fresh vapor will be rapidly formed, at a lower temperature, and, al- 

 though the expansive force of this diminishes in a geometric ratio, 

 yet it is still capable of opposing a resistance to the motion of the 

 piston. This resistance is such that it has been found by experience, 

 that the vapor of water at 212°, whose expansive force is equivalent 

 to a pi'essure of fifteen pounds on every square inch, had never acted 

 on the piston with a mean force of more than ten pounds, until means 

 were applied to remove or obviate this resistance. — p. 117. 



The reason why water boils at the temperature of 212° is that, 

 at that temperature, the vapor acquires just elasticity sufficient to 

 overcome the atmospheric pressure. Hence, it is said that steam 



