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 ...

FURNACE. 127
ciently rapid for the purposes of the engine. This apparatus
is called the self-acting dramper.
(72.) It has been proposed to connect this damper with
the safety valve invented by the Chevalier Edelcrantz. A
small brass cylinder is fixed to the boiler, and is fitted with
a piston which moves in it, without much friction, and nearly
steam-tight. The cylinder is closed at top, having a hole
through which the piston-rod plays; so that the piston is
thus prevented from being blown out of the cylinder by the
steam. The side of the cylinder is pierced with small holes
opening into the air, and placed at short distances above
each other. Let the piston be loaded with a weight proportional to the pressure of the steam intended to be produced.
When the steam has acquired a sufficient elasticity, the
piston will be lifted, and steam will escape through the first
hole. If the production of steam be not too rapid, and that
its pressure be not increasing, the piston will remain suspended in this manner: but if it increase, the piston will be raised
above the second hole, and it will continue to rise until the
escape of the steam through the holes is sufficient to render
the weight of the piston a counterpoise for the steam. This
safety valve is particularly well adapted to cases where steam
of an exactly uniform pressure is required; for the pressure
must necessarily be always equal to the weight on the piston.
Thus, suppose the section of the piston be equal to a square
inch; if it be loaded with 1Olbs., including its own weight,
the steam which will sustain it in any position in the cylinder, whether near the bottom or top, must always be exactly
equal in pressure to 10lbs. per inch. In this respect it
resembles the quality already explained in the governor, and
renders the pressure of the steam uniform, exactly in tile
same manner as the governor renders the velocity of the
engine uniform.
(73.) The economy of fuel depends, in a great degree, on
the construction of the furnace, independently of the effects
of the arrangements we have already described.