518 THE STEAM-ENGINE. 



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 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 : 



1. 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, 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 fric- 

 tion of the pipes and tubes through which it moves, should be taken into ac- 

 count 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. 



2. The radiation of heat from the cylinder and its appendages, will cause 

 a partial condensation of steam, and thereby produce a diminished mechanical 

 effect. 



3. The steam, which at each stroke of the piston fills the passages be- 

 tween the steam-valves and the piston, at the moment the latter commences 

 the stroke will be inefficient. If it were possible for the piston to come into 

 steam-tight contact with each end of the cylinder, and that the steam-valve 

 should be in immediate contact with the side or top of the piston, then the 

 whole of the steam which would pass through the steam-valve would be effi- 

 cient ; but as some space, however small, must remain between the piston and 

 the ends of the cylinder, and between the side of the cylinder and the steam- 

 valve, there will always be a volume of steam bearing a sensible proportion to 

 the magnitude of the cylinder, which at each stroke of the piston will be inef- 

 ficient. This volume of steam is called the clearance. 



4. Since the piston must move in steam-tight contact with the cylinder, 

 it must have a definite amount of friction with the sides of the cylinder by 

 whatever means it may be packed. This friction will produce a correspond- 

 ing resistance to the moving power. 



5. The various joints of ihe machinery where steam is contained are sub- 

 ject to leakage, and whatever amount of steam shall thus escape must be placed 

 to the account of power lost. 



6. When the eduction-valve is opened to admit the steam to the condenser, 

 * certain force is required to expel the steam from the cylinder. This force 

 reacts upon the piston, and counteracts to a proportional extent the moving 

 power of the steam on the other side. Besides this the water in the conden- 

 ser cannot be conveniently reduced below the temperature of about 100, and 



