would be unaffected by any such disturbing causes, and would perform with
little expense the work of many men.

162. _Q._--The introduction of some mechanical method of feeding the fire
with coals would enable a double tier of furnaces to be adopted in steam
vessels without inconvenience?

_A._--Yes, it would have at least that tendency; and as the space available
for area of grate is limited in a steam vessel by the width of the vessel,
it would be a great convenience if a double tier of furnaces could be
employed without a diminished effect. It appears to me, however, that the
objection would still remain of the steam raised by the lower furnace being
cooled and deadened by the air entering the ash-pit of the upper fire, for
it would strike upon the metal of the ash-pit bottom.

163. _Q._--Have any other plans been devised for feeding the fire by
self-acting means besides that of a revolving grate?

_A._--Yes, many plans, but none of them, perhaps, are free from an
objectionable complication. In some arrangements the bars are made like
screws, which being turned round slowly, gradually carry forward the coal;
while in other arrangements the same object is sought to be attained by
alternately lifting and depressing every second bar at the end nearest the
mouth of the furnace. In Juckes' furnace, the fire bars are arranged in the
manner of rows of endless chains working over a roller at the mouth of the
furnace, and another roller at the farther end of the furnace. These
rollers are put into slow revolution, and the coal which is deposited at
the mouth of the furnace is gradually carried forward by the motion of the
chains, which act like an endless web. The clinkers and ashes left after
the combustion of the coal, are precipitated into the ash-pit, where the