r; and in the
larger class of boilers, such as those suitable for 30 and 45 horse power
engines, the capacity of the steam room does not fall below this amount,
and, indeed, is nearer 6 than 5-3/4 cubic feet per horse power. The content
of water is 18-1/2 cubic feet per horse power in the two horse power
boiler, and 15 cubic feet per horse power in the 20 horse power boiler.

291. _Q._--Is this the proportion Boulton and Watt allow in their marine
boilers?

_A._--Boulton and Watt in their early steam vessels were in the habit of
allowing for the capacity of the steam, space in marine boilers 16 times
the content of the cylinder; but as there were two cylinders, this was
equivalent to 8 times the content of both cylinders, which is the
proportion commonly followed in land engines, and which agrees very nearly
with the proportion of between 5 and 6 cubic feet of steam room per horse
power already referred to. Taking for example an engine with 23 inches
diameter of cylinder and 4 feet stroke, which will be 18.4 horse power--the
area of the cylinder will be 415.476 square inches, which, multiplied by
48, the number of inches in the stroke, will give 19942.848 for the
capacity of the cylinder in cubic inches; 8 times this is 159542.784 cubic
inches, or 92.3 cubic feet; 92.3 divided by 18.4 is rather more than 5
cubic feet per horse power.

292. _Q._--Is the production of the steam in the boiler uniform throughout
the stroke of the engine?

_A._--It varies with the slight variations in the pressure within the
boiler throughout the stroke. Usually the larger part of the steam is
produced during the first part of the stroke of the engine, for there is
then the largest demand for steam, as the steam being commonly cut off
somewhat before the end of the stroke, the pressure rises somewhat in the
boiler during that period, and little steam is then produced. There is less
necessity that the steam space should be large when the flow of steam from
the boiler is very uniform, as it will be where there are two engines
attached to the boiler at right angles with one another, or where the
engines work at a great speed, as in the case of locomotive engines. A high
steam chest too, by rendering boiling over into the steam pipes, or priming
as it is called, more difficult, obviates the necessity for so large a
steam space; as does also a perforated steam pipe stretching through the
length of the boiler, so as not to take the steam from o