420 



THE STEAM-ENGINE. 



let the piston be counterbalanced by a weight W acting over a pulley, which 

 will be just sufficient to counterpoise the weight of the piston, so as leave no 

 force tending to keep the piston down, except the force of the atmosphere 

 acting above it. Under the circumstances here supposed, the piston being in 

 contact with the water, and all air being excluded, it will be pressed down by 

 the weight of the atmosphere, which we will suppose to be fifteen pounds, the 

 magnitude of the piston being a square inch. 



Now let the flame of a lamp be applied at the bottom of the tube ; the water 

 under the piston having its temperature thereby gradually raised, and being 

 submitted to no pressure save that of the atmosphere above the piston, it will 

 begin to be converted into steam when it has attained the temperature of 212°. 

 According as it is converted into steam, it will cause the piston to ascend in 

 the tube until all the water has been evaporated. If the tube were constructed 

 of sufficient length, the piston then would be found to have risen to the height 

 of about seventeen hundred inches, or one hundred and forty-two feet ; since, 

 as has been already explained, water passing into steam under the ordinary 

 pressure of the atmosphere undergoes an increase of bulk in the proportion of 

 about seventeen hundred to one. 



Now in this process, the air above the piston, which presses on it with a 

 force equal to fifteen pounds, has been raised one hundred and forty-two feet. 

 It appears, therefore, that, by the evaporation of a cubic inch of water under a 

 pressure equal to fifteen pounds per square inch, a mechanical force of this 

 amount is developed. 



It is evident that fifteen pounds raised one hundred and forty-two feet suc- 

 cessively, is equivalent to one hundred and forty -two times fifteen pounds 

 raised one foot. Now, one hundred and forty-two times fifteen is two thousand 

 one hundred and thirty, and therefore the force thus obtained is equal to two 

 thousand one hundred and thirty pounds raised one foot high. This being 

 within about 110 pounds of a ton, it may be stated, in round numbers, that, by 

 the evaporation of a cubic inch of water under these circumstances, a force is 

 obtained equal to that which would raise a ton weight a foot high. 



The augmentation of volume which water undergoes in passing into steam 

 under the pressure here supposed, may be easily retained in the memory, from 

 the accidental circumstance that a cubic inch of water is converted into a cubic 

 foot of steam, very nearly. A cubic foot contains one thousand seven hundred 

 and twenty-eight cubic inches — which is little different from the proportion 

 which steam bears to water, when raised under the atmosphoric pressure. 



It will, therefore, be an advantage to retain in memory the following general 

 facts : — 



1. A cubic inch of water evaporated under the ordinary atmospheric pres- 

 sure, is converted into a cubic foot of steam. 



2. A cubic inch of water evaporated under the atmospheric pressure, gives a 

 mechanical force equal to what would raise about a ton weight afoot high. 



Let us, again, suppose the piston P (fig. 5) to be restored to its original 

 position, with the liquid water beneath it ; and, in addition to the weight of 

 the atmosphere which before pressed it down, let us suppose another weight 

 of fifteen pounds laid upon it, so that the water below shall be pressed by 

 double the weight of the atmosphere. If the lamp were now applied, and at 

 the same time a thermometer were immersed in the water, it would be found 

 that the water would not begin to be converted into steam until it attained the 

 temperature of about 250°. The piston would then begin, as before, to ascend, 

 and the water to be gradually converted into vapor. The water being com- 

 pletely evaporated, it would be found that the piston would be raised to a height 

 little more than half its former height, or 72 feet. The mechanical effect, 



