ne place. The use
of steam of a high pressure, worked expansively, has the same operation; so
that in modern marine boilers, of the tubular construction, where the whole
or most of these modifying circumstances exist, there is no necessity for
so large a proportion of steam room as 5 or 6 cubic feet per nominal horse
power, and about one, 1-1/2, or 2 cubic feet of steam room per cubic foot
of water evaporated, more nearly represents the general practice.

293. _Q._--Is this the proportion of steam room adopted in locomotive
boilers?

_A._--No; in locomotive boilers the proportion of steam room per cubic foot
of water evaporated is considerably less even than this. It does not
usually exceed 1/5 of a cubic foot per cubic foot of water evaporated; and
with clean water, with a steam dome a few feet high set on the barrel of
the boiler, or with a perforated pipe stretching from end to end of the
barrel, and with the steam room divided about equally between the barrel
and the fire box, very little priming is found to occur even with this
small proportion of total steam room. About 3/4 the depth of the barrel is
usually filled with water, and 1/4 with steam.

294. _Q._--What is priming?

_A._--Priming is a violent agitation of the water within the boiler, in
consequence of which a large quantity of water passes off with the steam in
the shape of froth or spray. Such a result is injurious, both as regards
the efficacy of the engine, and the safety of the engine and boiler; for
the large volume of hot water carried by the steam into the condenser
impairs the vacuum, and throws a great load upon the air pump, which
diminishes the speed and available power of the engine; and the existence
of water within the cylinder, unless there be safety valves upon the
cylinder to permit its escape, will very probably cause some part of the
machinery to break, by suddenly arresting the motion of the piston when it
meets the surface of the water,--the slide valve being closed to the
condenser before the termination of the stroke, in all engines with lap
upon the valves, so that the water within the cylinder is prevented from
escaping in that direction. At the same time the boiler is emptied of its
water too rapidly for the feed pump to be able to maintain the supply, and
the flues are in danger of being burnt from a deficiency of water above
them.

295. _Q._--What are the causes of priming?

_A._--The causes of priming are an insufficien