(168.)

The gross effect of the moving power in a steam-engine, is the whole mechanical force developed by the evaporation [Pg287] of water in the boiler. A part of this effect is lost by the partial condensation of the steam before it acts upon the piston, and by the imperfect condensation of it subsequently: another portion is expended on overcoming the friction of the different moving parts, and in acting against the resistance which the air opposes to the machine. If the motion be subject to sudden shocks, a portion of the power is then lost by the destruction of momentum which such shocks produce. But if those parts of the machine which have a reciprocating motion be, as they ought to be, brought gradually to rest at each change of direction, then no power is absorbed in this way.