f 2136 square
feet, and a total heating surface of 2290 square feet. The weight of this
engine is stated to be 35 tons when ready to proceed on a journey. Both
engines were displayed at the Great Exhibition in 1851, as examples of the
most powerful locomotive engines then made. The weight of such engines is
very injurious to the railway; bending, crushing, and disturbing the rails,
and trying very severely the whole of the railway works. No doubt the
weight may be distributed upon a greater number of wheels, but if the
weight resting on the driving wheels be much reduced, they will not have
sufficient bite upon the rails to propel the train without slipping. This,
however, is only one of the evils which the demand for high rates of speed
has produced. The width of the railway, or, as it is termed, the _gauge_ of
the rails, being in most of the railways in this kingdom limited to 4 feet
8-1/2 inches, a corresponding limitation is imposed on the diameter of the
boiler; which in its turn restricts the number of the tubes which can be
employed. As, however, the attainment of a high rate of speed requires much
power, and consequently much heating surface in the boiler, and as the
number of tubes cannot be increased without reducing their diameter, it has
become necessary, in the case of powerful engines, to employ tubes of a
small diameter, and of a great length, to obtain the necessary quantity of
heating surface; and such tubes require a very strong draught in the
chimney to make them effective. With a draught of the usual intensity the
whole of the heat will be absorbed in the portion of the tube nearest the
fire box, leaving that portion nearest the smoke box nothing to do but to
transmit the smoke; and with long tubes of small diameter, therefore, a
very strong draught is indispensable. To obtain such a draught in
locomotives, it is necessary to contract the mouth of the blast pipe,
whereby the waste steam will be projected into the chimney with greater
force; but this contraction involves an increase of the pressure on the
eduction side of the piston, and consequently causes a diminution in the
power of the engine. Locomotives with small and long tubes, therefore, will
require more coke to do the same work than locomotives in which larger and
shorter tubes may be employed.

CHAPTER II.

HEAT, COMBUSTION, AND STEAM.

HEAT.

134. _Q._--What is meant by latent heat?

_A._--By latent heat is meant the heat e