326 Review of Renwick 
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 use of the steam engine, without understanding very 
fully the nature of the moving force. Let us therefore review the 
LEADING FACTS RESPECTING STEAM, especially such. as relate to the 
subject before us. | ty, Se = ™ 
‘1. It will be recollected, that the great and peculiar property, on 
which the meclranical 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 
he piston, which moves in the main Cylinder, raises it, and fills the 
space below it with steam. The steam is suddenly condensed, and j 
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 partsofthe engine. oi tig 
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. — saegied 
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 temper 
ture becomes lower, until, in the vacuum of an air pump, it boils at 
90°, [70°2]. Hence, so soon.as a portion of the steam is condensed, 
fresh vapor will be rapidly formed, at a lower temperature, and, aad 
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 expe?! 
that the vapor of water at 212°, whose expansive force is equivalent 
to a pressure of fifteen pounds on every square inch, had never acted 
on the piston with a mean foree of more than ten pounds, until means 
were applied ‘to remove or obviate this resistance.—p. 117. es 
_ The reason why water boils at the temperature of 212° is that, 
at that temperature, the vapor acquires just elasti 
overcome the atmospheric pressure. Hence, it is said that steam 
quae % 
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ence, 
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city sufficient 10 
