ON RAIL-ROADS. 285

relative performance, with respect to work,will be the same.

Yet, however obvious it may be, that, at aless rate of speed, the engines can travel withthe same load, it will not perhaps be equallyobvious, that at a greater rate these engineswill also travel with that load; and, to bring itto the apprehension of the reader, will requirea little explanation.

We have before seen that the pressure uponthe piston depended upon the density of thesteam in the boiler, and the velocity of effluxto the cylinder, regulated by the area of theaperture through which it issues: and expe-riment shewed, that when the piston was mov-ing at a less rate, that the steam acted witha greater force upon it, when the density in theboiler was the same. We will suppose, then,that it requires a certain number of cylindersfull of such a density to propel the enginewith 4-fcet wheels one mile, and that theboiler is capable of furnishing 90 cylindersfull, viz.

45 double strokes in a minute; now, as in each doublestroke the wheel makes one revolution, then it will make 45revolutions in a minute, and the progressive motion of theengine is 4 X 3.1416— 12.5 feet in a revolution, or 12.5