power applied to the axle is
carried at once to the parts of the wheels of least stress--the
circumferences. By this artifice the wheels are required to be of no
greater strength and weight than ordinary carriage-wheels; and, like
them, they turn freely and independently on the axle; but one or
both may be secured as part and parcel of the axle, as circumstances
require. The carriage is consequently propelled by the friction or
hold which either or both hind-wheels, according as the power is
applied to them jointly or separately, have on the ground. Beneath
the hind part drop two irons, with flat feet, called 'shoe-drags.' A
well-contrived apparatus, with a spindle passing up through a hollow
cylinder, to which the guiding handle is affixed, enables the director
to force one or both drags tight on the road, so as to retard the
progress in a descent, or if he please, to raise the wheels off
the ground. The propulsive power of the wheels being by this means
destroyed, the carriage is arrested in a yard or two, though going at
the rate of eighteen or twenty miles an hour. On the right hand of the
director lies the handle of the throttle-valve, by which he has the
power of increasing or diminishing the supply of steam _ad libitum_,
and hence of retarding or accelerating the carriage's velocity. The
whole carriage and machinery weigh about 16 cwt., and with the full
complement of water and coke 20 or 22 cwt., of which, I am informed,
about 16 cwt. lie on the hind-wheels."

Mr. H. then enumerates the principle of the improvements:--"That
troublesome appendage the fly-wheel, as I have observed, Mr. Gurney
has rendered unnecessary. The danger to be apprehended in going over
rough pitching, from too rapid a generation of steam, he avoids by a
curious application of springs; and should these be insufficient, one
or two safety valves afford the _ultimatum_ of security. He ensures
an easy descent down the steepest declivity by his 'shoe-drags,' and
the power of reversing the action of the engines. His hands direct,
and his foot literally pinches obedience to the course over the
roughest and most refractory ground. The dreadful consequences of
boiler-bursting are annihilated by a judicious application of tubular
boilers. Should, indeed, a tube burst, a hiss about equal to that of a
hot nail plunged into water, contains the sum total of alarm, while a
few strokes with a hammer will set all to rights again. Lastly, he has
so contrive