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 ...

150 THE STEAM ENGINE.
proportionate to the strength of the steam with which it is
proposed to work.
In the engines now described, this valve was frequently
loaded at the rate of from 60 to 80 lbs. on the square inch.
As the boilers of high-pressure engines were considered
more liable to accidents from bursting than those in which
steam of a lower pressure was used, greater precautions were
taken against such effects. A second safety valve was provided, which was not left in the power of the engine-man.
By this means he had a power to diminish the pressure of
the steam, but could not increase it beyond the limit determined by the valve, which was removed from his interference.
The greatest cause of danger, however, arose from the water
in the boiler being consumed by evaporation faster than it
was-supplied, and therefore falling below the level of the tube
containing the furnace. To guard against accidents arising
from this circumstance, a hole was bored in the boiler, at a
certain depth, below which the water should not be allowed
to fall; and in this hole a plug of metal was soldered with
lead, or with some other metal, which would fuse at that
temperature which would expose the boiler to danger. Thus,
in the event of the water being exhausted, so that its level
would fall below the plug, the heat of the furnace would
immediately melt the solder, and the plug would fall out,
affording a vent for the steam, without allowing the boiler to
burst. The mercurial steam gauge, already described, was
also used as an additional security. When the force of the
steam exceeded the length of the column of mercury which
the tube would contain, the mercury would be blown out,
and the tube would give vent to the steam. The water by
which the boiler was replenished was forced into it by a
pump worked by the engine. In order to economize the
heat, this water was contained in a tube T. which surrounded
the pipe IT. As the waste steam, after working the piston,
passed off through ir, it imparted a portion of its heat to the