On the other hand, supposing that no change took place in the amount of the load driven by the engine, and that the same number of machines of whatever kind would have to be continually driven, the motion imparted to the main axle would still be subject to variation by the changes inevitable to the moving power. The piston of the engine being subject to an unvaried resistance, a uniform motion could only be imparted to it, by maintaining a corresponding uniformity in the impelling power. This would require a uniform supply of steam from the boiler, which would further imply a uniform rate of evaporation in the boiler, unless means were provided in the admission of steam from the boiler to the cylinder to prevent any excess of steam which might be produced in the boiler from reaching the cylinder. [Illustration: _Fig._ 39.] [Illustration: _Fig._ 40.] This end was attained by a contrivance afterwards called the _throttle-valve_. An axis A B (_figs._ 39, 40.) was placed across the steam pipe in a ring of cast-iron D E, of proper thickness. On this axis was fastened a thin circular plate T, of nearly the same diameter as the steam pipe. On the outer end B of this axle was placed a short lever or handle B C, by which it could be turned. When the circular plate T was turned into such a position as to be at right angles to the length of the tube, it stopped the passage within the tube altogether, so that no steam could pass from the boiler to the engine. On the other hand, when the handle was turned through a fourth of a revolution from this position, then the circular plate T had its plane in the direction of the length of the tube, so that its edge would be presented towards the current of steam flowing from the boiler to the cylinder. In that position the passage within the tube [Pg208] would be necessarily unobstructed by the throttle-valve. In intermediate positions of the valve, as that represented in _figs._ 39, 40., the passage might be left more or less opened, so that steam from the boiler might be admitted to the cylinder in any regulated quantity according to the position given to the lever B C. A view of the throttle-valve taken by a section across the steam pipe is exhibited in _fig._ 40., and a section of it through the axis of the steam pipe is represented in _fig._ 39. The form of the valve is such, that, if accurately constructed, the steam in passing from the boiler would have no effect by its pressure to alter any position which might be given to the valve; and any slight inaccuracy of form which might give a tendency to the steam to alter the position would be easily counteracted by the friction of the valve upon its axle. The latter might be regulated at pleasure.