(173.) The quantity of power expended in working the engine itself, independently of that required to move its load, will be less in proportion to the degree of perfection which [Pg290] may be attained in the construction of the engine, and to the order in which it is kept while working. Engines vary one from another so much in these respects, that it is scarcely possible to lay down any general rules for the quantity of power to be allowed over and above what is necessary to move the load. The means whereby mechanical power is expended in working the engine may be enumerated as follows:— _First._ Steam in passing from the boiler to the cylinder is liable to lose its temperature by the radiation of the steam-pipes and other passages through which it is conducted. Since the steam produced in the boiler is in contact with water, it will be common steam (94.), and consequently the least loss of heat will cause a partial condensation. To whatever extent this condensation may be carried, a proportional loss of power, in reference to the heat obtained from the fuel, will be entailed upon the engine. It has been said that the force necessary to move the steam from the boiler to the cylinder through passages more or less contracted, subject to the friction of the pipes and tubes through which it moves, should be taken into account in estimating the power, and a corresponding deduction made. This, however, is not the case: the steam having passed into the cylinder remains common steam, its pressure being diminished by reason of the force expended in thus moving it from the boiler to the cylinder. But its mechanical efficacy at the reduced pressure is not sensibly different from the efficacy which it had in the boiler. If at the reduced pressure its volume were the same, then a loss of effect would be sustained equivalent to the difference of the pressures; but its volume being augmented in very nearly the same proportion as its pressure is diminished, the mechanical efficacy of a given weight of steam in the cylinder will be sensibly the same as in the boiler. _Second._ The radiation of heat from the cylinder and its appendages, will cause a partial condensation of steam, and thereby produce a diminished mechanical effect.