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 ...

BOILER AND ITS APPENDAGES. 119
pins P P assume the position in fig. 34. Thus, in general,
the position of the pins P P' becomes an indication of the
quantity of water in the boiler.
Another method is to place a glass tube (fig. 35) with one
end T entering the boiler above the proper level, and the
other end T' entering it below the proper level. It must
be evident that the water in the tube will always stand at
the same level as the water in the boiler; since the lower
part ha's a free communication with that water, while the
surface is submitted to the pressure of the same steam as
the water in the boiler. This, and the last-mentioned
gauge, have the advantage of addressing the eye of the
engineer at once, without any adjustment; whereas the
gauge cocks must be both opened, whenever the depth is to
be ascertained.
These gauges, however, require the frequent attention of
the engine-man; and it becomes desirable either to find
some more effectual means of awakening that attention, or to
render the supply of the boiler independent of any attention.
In order to enforce the attention of the engine-man to replenish the boiler when partially exhausted by evaporation, a
tube was sometimes inserted at the lowest level to which
it was intended that the water should be permitted to fall.
This tube was conducted from the boiler into the enginehouse, where it terminated in a mouth-piece or whistle, so
that, whenever the water fell below the level at which this
tube was inserted in the boiler, the steam would rush through
it, and, issuing with great velocity at the mouth-piece, would
summon the engineer to his duty with a call that would rouse
him even from sleep.
(67.) In the most effectual of these methods, the task of
replenishing the boiler should still be executed by the
engineer; and the utmost that the boiler itself was made
to do was to give due notice of the necessity for the supply
of water. The consequence was, among other inconve