ed by
referring to the effect of contracting the pipe of a water-hose, by which
the force of the jet of water is proportionately increased. Widen the
nozzle of the pipe, and the force is in like manner diminished. So is it
with the steam-blast in the chimney of the locomotive.

Doubts were, however, expressed whether the greater draught secured by
the contraction of the blast-pipe was not counterbalanced in some degree
by the negative pressure upon the piston. A series of experiments was
made with pipes of different diameters; the amount of vacuum produced
being determined by a glass tube open at both ends, which was fixed to
the bottom of the smoke-box, and descended into a bucket of water. As
the rarefaction took place, the water would of course rise in the tube;
and the height to which it rose above the surface of the water in the
bucket was made the measure of the amount of rarefaction. These
experiments proved that a considerable increase of draught was obtained
by the contraction of the orifice; accordingly, the two blast-pipes
opening from the cylinders into either side of the "Rocket" chimney, and
turned up within it, were contracted slightly below the area of the
steam-ports; and before the engine left the factory, the water rose in
the glass tube three inches above the water in the bucket.

[Picture: The "Rocket"]

The other arrangements of the "Rocket" were briefly these:--the boiler
was cylindrical with flat ends, 6 feet in length, and 3 feet 4 inches in
diameter. The upper half of the boiler was used as a reservoir for the
steam, the lower half being filled with water. Through the lower part,
25 copper tubes of 3 inches diameter extended, which were open to the
fire-box at one end, and to the chimney at the other. The fire-box, or
furnace, 2 feet wide and 3 feet high, was attached immediately behind the
boiler, and was also surrounded with water. The cylinders of the engine
were placed on each side of the boiler, in an oblique position, one end
being nearly level with the top of the boiler at its after end, and the
other pointing towards the centre of the foremost or driving pair of
wheels, with which the connection was directly made from the piston-rod,
to a pin on the outside of the wheel. The engine, together with its load
of water, weighed only 4.25 tons, and was supported on four wheels, not
coupled. The tender was four-wheeled, and similar in shape to a
waggon,--t