The steam engine, steam navigation, roads, and railways, explained and illustrated by Lardner Dionysius 1793-1859

Author:Lardner, Dionysius, 1793-1859
Language: eng
Format: epub, pdf
Tags: Steam-engines
Publisher: London Taylor, Walton, and Maberly
Published: 1851-03-25T05:00:00+00:00

will suppose to be 15 lbs., 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 sufiicient length, the piston then would be found to have risen to the height of about 1700 inches, or 142 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.

In this process, the air above the piston, which presses on it with a force equal to 15 lbs., has been raised 142 feet. It appears, therefore, that, by the evaporation of a cubic inch of water under a pressure equal to 15 lbs. per square inch, a mechanical force of this amount is developed.

It is evident that 15 lbs. raised 142 feet successively, is equivalent to one hundred and forty-two times 15 lbs. 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 2130 lbs. raised one foot high. This being within about 110 lbs. 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 1728 cubic inches, which is little different from the proportion which steam bears to water, when raised under the atmospheric pressure.

(137.) Effect produced by evaporating a given quantity of water. — It will, therefore, be an advantage to retain in memory the following general facts : —

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