169. _Q._--If this be so, the quantity of heat in a given weight of steam
must be nearly the same, whether the steam is high or low pressure?

_A._--Yes; the heat in steam is nearly a constant quantity, at all
pressures, but not so precisely. Steam to which an additional quantity of
heat has been imparted after leaving the boiler, or as it is called
"surcharged steam," comes under a different law, for the elasticity of such
steam may be increased without any addition being made to its weight; but
surcharged steam is not at present employed for working engines, and it may
therefore be considered in practice that a pound of steam contains very
nearly the same quantity of heat at all pressures.

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