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. 121
came, bringing back all the unconsumed portion of its heat
preparatory to being once more put in circulation through
the machine.
The entire quantity of hot water pumped into the cistern
c is not always required for the boiler. A waste pipe may
be provided for carrying off the surplus, which may be
turned to any purpose for which it may be required; or it
may be discharged into a cistern to cool, preparatory to
being restored to the cold cistern, (fig. 12,) in case water
for the supply of that cistern be not sufficiently abundant.
In cities and places in which it becomes an object to prevent the waste of water, the waste pipes proceeding from the
feed cistern c, (fig. 36,) and from the cold cistern containing
the condenser and air-pump, may be conducted to a cistern
A B, (fig. 27.) Let c be the pipe from the feeding cistern,
and D that from the cold cistern; by these pipes the waste
water from both these cisterns is deposited in A B. In the
bottom of A B is a valve v, opening upward, connected with
a float r. When fhe quantity of water collected in the cistern A B is such that the level rises considerably, the float F
is raised, and lifts the valve v, and the water flows into the
main pipe, which supplies water for working the engine: G
is the cold water pump for the supply of the cold cistern.
This arrangement for saving the water discharged from
the feeding and condensing cisterns has been adopted in the
printing office of the Bank of Ireland, and a very considerable waste of water is thereby prevented.
(68.) It is necessary to have a ready method of ascertaining at all times the strength of the steam which is used in
working the engine. For this purpose a bent tube containing mercury is inserted into some part of the apparatus
which has free communication with the steam. It is usually
inserted in the jacket of the cylinder, (44.) Let A B c (fig.
38) be such a tube. The pressure of the steam forces the
mercury down in the leg A B and up in the leg n c. If the
mercury in both legs be at exactly the same level, the pres
L 16