ON RAIL-ROADS.

269

steam produced;—and both these operate inbringing the engine into a state of uniformvelocity.

We might indeed, by enlarging the aperturebetween the boiler and the cylinder, increasethe velocity of efflux, and consequently that ofthe piston ; but this would again be renderedfutile if the supply of steam could not be keptup ; the latter will, therefore, be the principalregulator of the velocity.

We thus see why the same engine is capableof producing very different effects, when em-ployed in dragging different weights. In thecase of the high-pressure engine, ExperimentXXVI., when dragging 8 and 9 carriages, theperformance was as 26.7 : 30, but the time wasproportionably diminished, and from the causesabove stated. The pressure in the boiler wasthe same in each case, but the elasticity of thesteam in the cylinder being regulated by theweight upon the piston, the rate of efflux wouldbe thus diminished, and the density whenmoving slow would, therefore, be proportion-ably increased. Taking the elasticity of steamin the boiler as constant, and applying this tothe area of the piston, the performances willappear different; but, in such cases, the weightof the steam, which is the true measure of