170. Q.—­Does not the quantity of heat in any body vary with the temperature?

A.—­Other circumstances remaining the same the quantity of heat in a body increases with the temperatures.

171. Q.—­And is not high pressure steam hotter than low pressure steam?

A.—­Yes, the temperature of steam rises with the pressure.

172. Q.—­How then comes it, that there is the same quantity of heat in the same weight of high and low pressure steam, when the high pressure steam has the highest temperature?

A.—­Because although the temperature or sensible heat rises with the pressure, the latent heat becomes less in about the same proportion.  And as has been already explained, the latent and sensible heats taken together make up nearly the same amount at all temperatures; but the amount is somewhat greater at the higher temperatures.  As a damp sponge becomes wet when subjected to pressure, so warm vapor becomes hot when forced into less bulk, but in neither case does the quantity of moisture or the quantity of heat sustain any alteration.  Common air becomes so hot by compression that tinder may be inflamed by it, as is seen in the instrument for producing instantaneous light by suddenly forcing air into a syringe.

173. Q.—­What law is followed by surcharged steam on the application of heat?

A.—­The same as that followed by air, in which the increments in volume are very nearly in the same proportion as the increments in temperature; and the increment in volume for each degree of increased temperature is 1/490th part of the volume at 32 deg..  A volume of air which, at the temperature of 32 deg., occupies 100 cubic feet, will at 212 deg. fill a space of 136.73 cubic feet.  The volume which air or steam—­out of contact with water—­of a given temperature acquires by being heated to a higher temperature, the pressure remaining the same, may be found by the following rule:—­To each of the temperatures before and after expansion, add the constant number 458:  divide the greater sum by the less, and multiply the quotient by the volume at the lower temperature; the product will give the expanded volume.

174. Q.—­If the relative volumes of steam and water are known, is it possible to tell the quantity of water which should be supplied to a boiler, when the quantity of steam expended is specified?

A.—­Yes; at the atmospheric pressure, about a cubic inch of water has to be supplied to the boiler for every cubic foot of steam abstracted; at other pressures, the relative bulk of water and steam may be determined as follows:—­To the temperature of steam in degrees of Fahrenheit, add the constant number 458, multiply the sum by 37.3, and divide the product by the elastic force of the steam in pounds per square inch; the quotient will give the volume required.

175. Q.—­Will this rule give the proper dimensions of the pump for feeding the boiler with water?