ed whether the greater draught obtained by
the contraction of the blast-pipe was not counterbalanced in some degree
by the negative pressure upon the piston. Hence a series of experiments
was made with pipes of different diameters, and their efficiency was
tested by the amount of vacuum that was produced in the smoke-box. The
degree of rarefaction was determined by a glass tube fixed to the bottom
of the smoke-box and descending into a bucket of water, the tube being
open at both ends. 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.

The other arrangements of the "Rocket" were briefly these: the boiler
was cylindrical, with flat ends, six feet in length, and three feet four
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 the copper tubes extended, being open to the fire-box at one end,
and to the chimney at the other. The fire-box, or furnace, two feet wide
and three 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 toward 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 four tons and a quarter; and it was supported on
four wheels, not coupled. The tender was four-wheeled, and similar in
shape to a waggon--the foremost part holding the fuel, and the hind part
a water cask.

When the "Rocket" was finished it was placed upon the Killingworth
Railway for the purpose of experiment. The new boiler arrangement was
found perfectly successful. The steam was raised rapidly and