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. 



This end was attained by a contrivance afterward called the throttle-valve. 

 An axis A B figs. 20, 21, was placed across the steam-pipe in a ring of cast- 



ing. 21 



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 revolu- 

 tion 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 toward the cur- 

 rent of steam flowing from the boiler to the cylinder. In that position the 

 passage within the tube would be necessarily unobstructed by the throttle-valve. 

 In intermediate positions of the valve, as that represented in figs. 20, 21, 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 exhibit- 

 ed in fig. 21, and a section of it through the axis of the steam-pipe is represented 

 in fig. 20. 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. 



