amount of power spent in moving the engine, or which may be wasted or destroyed in any way by the engine.

It is usual to express and estimate all mechanical effect whatever by nature of the resistance overcome, by an equivalent weight raised a certain height. Thus, if an engine exerts a certain power in driving a mill, in drawing a carriage on a road, or in propelling a vessel on water, the resistance against which it has to act must be equal to a definite amount of weight. If a carriage be drawn, the traces are stretched by the tractive power, by the same tension that would be given to them if a certain weight were appended to them. If the paddle-wheels of a boat are made to revolve, the water opposes to them a resistance equal to that which would be produced, if instead of moving the water the wheel had to raise some certain weight. In any case, therefore, weight becomes the exponent of the energy of the resistance against which the moving power acts.

But the amount of mechanical effect depends conjointly on the amount of resistance, and the space through which that resistance is moved. The quantity of this effect, therefore, will be increased in the same proportion, whether the quantity of resistance or the space through which that resistance is moved be augmented. Thus, a resistance of one hundred pounds, moved through two feet, is mechanically equivalent to a resistance of two hundred pounds moved through one foot, or of four hundred pounds moved through six inches. To simplify, therefore, the expression of mechanical effect, it is usual to reduce it invariably to a certain weight raised one foot. If the resistance under consideration be equivalent to a certain weight raised through ten feet, it is always expressed by ten times the amount of that weight raised through one foot.

It has also been usual in the expression of mechanical ef fect, to take the pound weight as the unit of weight, and the foot as the unit of length, so that all mechanical effect whatsoever is expressed by a certain number of pounds raised one foot.

(168.) The gross effect of the moving power in a steamengine, is the whole mechanical force developed by the evapo

ration 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.

(169.) The useful effect of an engine is variously denominated according to the relation under which it is considered. If it be referred to the time during which it is produced, it is called POWER.

(170.) If it be referred to the fuel, by the combustion of which the evaporation has been effected, it is called DUTY.

(171.) When steam-engines were first brought into use, they were commonly applied to work pumps for mills which had been previously worked or driven by horses. In forming their contracts, the first steam-engine builders found themselves called upon to supply engines capable of executing the same work as was previously executed by some certain number of horses. It was therefore convenient, and indeed necessary, to be able to express the performance of these machines by comparison with the animal power to which manufacturers, miners, and others, had been so long accustomed. When an engine, therefore, was capable of performing the same work in a given time as any given number of horses of average strength usually performed, it was said to be an engine of so many horses' power. Steam-engines had been in use for a considerable time before this term had acquired any settled or uniform meaning, and the nominal power of engines was accordingly very arbitrary. At length, however, the use of steam-engines became more extended, and the confusion and inconvenience arising out of all questions respecting the per

and definite meaning should be assigned to the terms by which the powers of this machine were expressed. To have abandoned the term horse-power, which had been so long in use, would have been obviously inconvenient; nor could there be any objection to its continuance, provided all engine-makers, and all those who used engines, could be brought to agree upon some standard by which the unit of horse-power might be defined. The performance of a horse of average strength working for eight hours a day was therefore selected as a standard, or unit, of steam-engine power. Smeaton estimated that such an animal, so working, was capable of performing a quantity of work equal in its mechanical effect to 22,916 lbs. raised one foot per minute, while Desaguliers estimated the same power at 27,500 lbs. raised through the same height in the same time. The discrepancy between these estimates probably arose from their being made from the performances of different classes of horses. Messrs. Boulton and Watt caused experiments to be made with the strong horses used in the breweries in London, and from the result of these trials they assigned 33,000 lbs. raised one foot per minute, as the value of a horse's power. This is the unit of engine-power now universally adopted; and when an engine is said to be of so many horses' power, what is meant is, that that engine, in good working order and properly managed, is capable of moving a resistance equal to 33,000 lbs. through one foot per minute. Thus an engine of ten horse-power is one that would raise 330,000 lbs. weight one foot per minute.

Whether this estimate of an average horse's power be correct or not, in reference to the actual work which the animal is capable of executing, is a matter of no present importance in its application to steam-power. The steamengine is no longer used to replace the power of horses, and therefore no contracts are based upon such a comparison. The term horse-power, therefore, as applied to steam-engines, must be understood to have no reference whatever to the actual animal power, but must be taken as a term having no other meaning than the expression of the ability of the

machine to move the amount of resistance above mentioned through one foot per minute.

(172.) It has been already explained (67.) that the conversion of a given volume of water into steam is productive of a certain definite amount of mechanical force, this amount depending on the pressure under which the water is evaporated, and the extent to which the expansive principle is used in working the steam. It is evident that this amount of mechanical effect is a major limit, which cannot be exceeded by the power of the engine.

If the steam be not worked expansively, then the whole power of the water, transmitted in the form of steam from the boiler to the working machinery, will be a matter of easy calculation, when the pressure at which the steam is worked is known. A table, exhibiting the mechanical power of a cubic foot of water converted into steam at various pressures, expressed in an equivalent number of pounds' weight raised one foot high, is given in the Appendix to this volume. Where much accuracy is sought for, the pressure at which the steam is used must be taken into account; but by reference to the table it will be seen, that when steam is worked without expansion, its mechanical effect varies very little with the pressure. It may therefore be assumed, as has been already stated, that for every cubic inch of water transmitted in the form of steam to the cylinders, a force is produced, represented by a ton weight raised a foot high. Now, as 33,000 lbs. is very nearly 15 tons, it follows that 15 cubic inches of water converted into steam per minute, or 900 cubic inches per hour, will produce a mechanical force equal to one horse. If, therefore, to 900 cubic inches be added the quantity of water per hour necessary to move the engine itself, independently of its load, we shall obtain the quantity of water per hour which must be supplied by the boiler to the engine for each horse-power, and this will be the same whatever may be the magnitude or proportions of the cylinder.

(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

U

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 steampipes 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.

Third. The steam, which at each stroke of the piston fills the passages between the steam-valves and the piston, at the