as well as because the transport of a large quantity of cold water could not be conveniently effected, high-pressure engines alone are preferable to all others for locomotive purposes; and even with these, it is necessary to resort to extraordinary means to combine sufficient powers of steam for the loads that it is necessary to draw, with a sufficient heating power to produce that steam, in the quantity necessary to maintain the speed at which the engine is capable to travel.
A travelling steam-engine is placed like an ordinary carriage, upon four wheels. The axle of one pair of these wheels is furnished with cranks, as already described; which cranks or driving wheel are worked by the pistons of the cylinders of the engine, so as to keep the axles in a constant state of rotation. Upon this axle the wheels are fixed so as to be incapable of turning independent of the axle, as the wheels of a carriage do; consequently, when the engine causes the axle to revolve, it necessarily causes the wheels fixed upon that axle also to revolve. The pressure of the wheels upon the road gives them a certain degree of adhesion, so that they are incapable of slipping. When the axle is turned by the engine, the carriage must therefore advance as the wheels revolve. One stroke of the piston corresponds to one revolution of the wheels; and in one revolution of the wheels, the carriage advances through a space equal to their circumference; consequently every stroke of the piston propels the carriage along the road, through a space equal to the circumference of the working wheels. It is apparent, therefore, that the speed or rate of motion of the carriage will depend on the rate at which the
