An INDICATED horse-power is the same measure of work, but it is "indicated,” that is, measured or shown by an instrument inside the cylinder, and thus does not show what is available on the shaft or wheel. It is in effect the real power of the expanded steam, or the exploded gas or oil, and of course from it must be deducted the power it takes to push the working parts round, before the real work it represents is ascertained. This deduction may be averaged at fifteen per cent. In the calculations which follow it has been taken at as high as twenty per cent., for the sake of absolute security. The indicated horse-power of an engine is thus a most useful term, because it tells what work is being really accomplished by the expansion of the steam or gas, and further gives us, in the case of steam-engines, a measure of what the boiler is doing, or what it ought to do.

A "COMMERCIAL horse-power is a term which has been widely used in the United States, since its adoption as a standard of comparison by the judges at the Centennial Exhibition. It represents an amount of 30 pounds of water evaporated from feed water at a heat of 100° F. and raised therefrom to 70 pounds pressure. It is, of course, merely a selection of these figures out of all others, but, as far as it goes, is a reasonably average performance to select, and if it were universally used it would form a fair basis to work upon. Its application is practically suited only to boilers, although by proportionate calculation a parity can be established with steam-engines working under different conditions, but even as regards boilers, as their pressure varies, it becomes for all, except those suited to 70 pounds pressure, a merely theoretical basis.

It does not, therefore, possess sufficient merit as a term to warrant expectation of world-wide adoption, and for engines it will not supersede the effective horse-power, nor for boilers the basis of comparative heating and grate surfaces. It is, of course, manifest that mere heating surface with

out regard to its disposition or efficient position, is not a fair means of comparison between different types of boilers, but it nevertheless forms the only reasonable comparison between boilers of the same pattern.

A NOMINAL horse-power is a commercial term to which unfortunately a large number of manufacturers and merchants, especially in Great Britain, still cling, to denote the sizes of their fixed and portable engines, and especially of the latter.

It is a term of no value, nor of any fixed quantity. It is supposed to mean, in some cases, about one-fourth of what an engine will indicate, in other cases about one-third of the same.

The nominal horse-power of a boiler is not the same as that of an engine, in fact, it is a worthless and misleading term, and very doubtful use has frequently been made of it in covering deficiencies in engines and scamping of dimensions. Reference to Chapters XVIII. and XXIII. will make the above remarks quite clear, and afford material for dealing with any use or misuse of this term.

A RATED horse-power is a term only used in America, where it occupies much the same equivocal position as the term "Nominal" and has about the same negative value. Animal Powers.-The measure of power for machinery being thus established at 30,000 foot-pounds, we can compare with it the following muscular, or animal powers:

Thus the work of twelve men at cranks will only equal one effective horse-power.

Electric Powers.—The electricians have been fortunate, in the early stages of the development of their profession, in being able to settle clearly the terms for the definition of electric currents.

Four terms practically cover the ground.

Volt is a term used to define electric pressure, and is practically applied to the electric current as pressure per square inch is by engineers to steam. It is also known as electro-motive force; frequently written E. M. F. for convenience, also potential, and is constantly spoken of as tension or "voltage." This interchange of terms is to be regretted. Here we use exclusively the word volt and voltage.

Ampères are the quantity of current, and may be compared with the entire quantities used in defining steam or water. It is frequently written "current." As quantity multiplied by pressure gives us in other calculations a definition of power, so ampères multiplied by volts give us

Watts, or volt-ampères, which are practically the footpounds by which we define a horse-power. The Watt is an arbitrary quantity of 1 ampère at 1 volt, of which 746 equal a horse-power, and they constitute the means of comparing electric energy with other powers.

The Ohm is the term used to define the resistance of conductors or wires to the passage of electricity. It answers to the friction opposed to liquids passing through a pipe. The standard ohm is the resistance due to a copper wire of an inch diameter × 129 yards long. As every conductor offers some resistance to the flow of electricity, the larger the wire the less will be its resistance. Similarly the shorter the wire the less will be its resistance. In estimations of power of electric energy, it is always necessary to bear in mind those losses which occur in all mechanism, due to friction, imperfections, and leakage.

Thus, 10 effective horse-power employed to rotate a dynamo will not produce full 10 effective horse-power of electricity, but a less amount, which may safely be taken as eighty per cent., or 8 effective horse-power, and is so taken in the succeeding tables and calculations.

Inversely, 8 horse-power of electricity given out by a dynamo requires more than 8 effective horse-power to produce it.

Similarly, the conduction of the current over a wire involves a certain loss by friction, which must be allowed for, and of which tables are given, rendering elaborate calculation unnecessary.

Then, the supply of a given quantity of electricity, say at number of Watts, to a motor will not result in an exactly corresponding effective horse-power, but an amount less by from 10 to 15 per cent., which, in my tables, I have, for entire security, taken at 20 per cent.

SECTION II.

CHAPTER VI.

MANUAL POWER.

THE labour of man cannot be relied upon for long spells of heavy work. The estimate of Morin, that a man-power equalled 3,000 lbs. lifted i foot high in a minute, is only true of a very muscular specimen of the human race. The extremity of human exertion is developed in the act of rowing; in which art, enthusiasts are proud to claim that every muscle in the body is developed. At such a labour the maximum effort may reach 4,000 lbs. raised 1 foot high in 1 minute, and in the labour of the tread-wheel a man may reach 3,100 foot-pounds for a spell of work; but labour under such conditions is what humanity would decline to avail itself of, and we are reduced in the average hard work of a man to 2,600 to 2,750 lbs. raised 1 foot high in 1 minute. This is the sort of work developed in turning a crank, which is the most convenient form of application of manual power to machinery.

I

Crank Handles. Such cranks should be situated on a shaft about 3 feet from ground level, and should be about 16 inches long, or 32 inches diameter of path.

On these a man imparts a constant pressure of about 15 lbs., which for intermittent work may be increased towards 25 lbs.

The speed at which a man will turn a crank is from 26 to 30 revolutions per minute.