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 ...

228 THE STEAM ENGINE.
road, and consequently the propelling power, or the pressure
upon the pistons of the engine, must be susceptible of a core
responding variation; but a still greater variation becomes
necessary from the undulations and hills which are upon all
ordinary roads. This necessary change in the intensity of
the impelling power is obtained by restraining the steam in
the boiler by the throttle valve, as already described in the
locomotive engines on the railroad. This principle, however, is carried much further in the present case. The steam
in the boiler may be at a pressure of from 100 to 200 lbs.
on the square inch; while the steam on the working piston
may not exceed 30 or 40 lbs. on the inch. Thus an immense
increase of power is always at the command of the conductor;
so that when a hill is encountered, or a rough piece of road,
he is enabled to lay on power sufficient to meet the exigency
of the occasion.
The two difficulties which have been always apprehended
in the practical working of steam carriages upon common
roads are, first, the command of sufficient power for hills and
rough pieces of road; and, secondly, the apprehended insufficiency of the adhesion of the wheels with the road to
propel the carriage. The former of these difficulties has
been met by allowing steam of a very great pressure to be
constantly maintained in the boiler with perfect safety. As
to the second, all experiments tend to show that there is no
ground for the supposition that the adhesion of the wheels
is in any case insufficient for the purposes of propulsion.
Mr. Gurney states, that he has succeeded in driving carriages thus propelled up considerable hills on the turnpike
roads about London. He made a journey to Barnet with
only one wheel attached to the axle, which was found sufficient to propel the carriage up all the hills upon that road.
The same carriage, with only one propelling wheel, also
went to Bath, and surmounted all the hills between Cranford
Bridge and Bath, going and returning.
A double stroke of the piston produces one revolution of