Moreover, the method of heat economy is suited for
application to automatic generators rather than to those belonging to the
opposite system, because automatic apparatus will be generating gas, and
consequently evolving heat, every evening till late at night--just at the
time when frost begins to be severe. A non-automatic generator will
usually be at work only in the mornings, and its store of heat will
accordingly be much more difficult to retain till nightfall. With the
object of storing up the heat evolved in the generator, it must be
covered with some material possessed of the lowest heat-conducting power
possible; and the proper positions for that material in order of
decreasing importance are the top, sides, and bottom of the plant. The
generator may either be covered with a thick layer of straw, carpet,
flannel, or the like, as is done in the protection of exposed water-
pipes; or it may be provided with a jacket filled with some liquid. In
view of the advisability of not having any organic or combustible
material near the generator, the solid substances just mentioned may
preferably be replaced by one of those partially inorganic compositions
sold for "lagging" steam-pipes and engine-cylinders, such as "Fossil
meal.
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