The steam engine familiarly explained and illustrated; with an historical sketch of its invention and progressive improvement; its applications to navigation and railways; with plain maxims for railway speculators. By the Rev. Dionysius Lardner ... With additions and notes by James Renwick ...

POWER OF BOILERS. 299
contents of the cylinder. For example, if the volume of
steam in the boiler' were precisely equal to the capacity of
the cylinder, then one measure of the cylinder would for
the moment cause the steam to expand into double its bulk
and to lose half its force, supposing it to pass freely from the
boiler to the cylinder. In the same manner, if the volume
of steam contained in the boiler were twice the contents of
the cylinder, the steam would for a moment lose a third of
its force, and so on. It is clear, therefore, that the space
allotted to steam in the boiler must be so many times greater
than the magnitude of the cylinder, that the abstraction of
a cylinder full of steam from it shall cause a very trifling
diminution of its force.
In the same manner, we may perceive the necessity of
maintaining a large proportion between the total quantity of
water in the boiler, and the quantity supplied in the form of
steam to the cylinder. If, for example, (taking as before an
extreme case,) the quantity of water in the boiler were only
equal to the quantity supplied in the form of steam to the
cylinder in a minute, it would be necessary that the contents
of the boiler should be replaced by cold water once in each
minute: and, under such circumstances, it is evident that the
action of the heat upon the water would be quite unmanageable. But, independent of this, the quantity of water must
be sufficient to fill the boiler above the point at which the
flue surface terminates, otherwise the heat of the fuel would
act upon the part of the boiler containing steam and not
water; and, steam receiving heat sluggishly, the metal of the
boiler would be gradually destroyed by undue temperature.
The total quantity of space for water and steam in boilers
is subject to considerable variation in proportion to their
power. Small boilers require a greater propoition,f steam
and water room, or a greater capacity of boiler, in proportion,
than large ones; and the same applies to their fire surface and
flue surface.
Tihe general experience of engineers has led to the concli