In making an estimate or measure of the effect of any piece of mechanism that is used to overcome resistance, it is necessary to have a unit of reference. In whatever manner the resistance is overcome, if it can be measured it can be converted into the amount of work that must be expended to raise a weight through a distance, since by suitable arrangements the mechanism can be put in motion and made to overcome resistance by allowing the weight to fall. This gives a simple mode of estimating the work, by assuming that a unit of work is the amount required to raise 1 pound a distance of 1 foot vertically. To illustrate, suppose that a cut is being taken from a 6-in. shaft in a lathe, and that the resistance to the motion of the cutting-tool is 200 lbs.; how many units of work are performed each time the shaft makes a revolution?

In each revolution of the shaft, the tool makes a cut 6 times 3.1416, or 18.8496 ins., or 1.5708 ft. in length, and the work is the same as would be required to raise a weight of 200 lbs. through a vertical distance of 1.5708 ft., or it is 314.16 units.

Now, if 33, 000 units of work are performed in a minute, they constitute a unit of power, known as a horse-power--and conversely, a horse-power can be defined as the power required to raise 33, 000 lbs. 1 ft, high, or do 33, 000 units of work in a minute. Again, a horse-power may be defined as the power required to perform 550 units of work in a second, or 1, 980, 000 units in an hour. To apply the principle to the example given ahove, suppose the shaft makes 20 revolutions a minute, how many horse-power are required to drive the tool? In this case, the units of work performed per minute would he 20 times 314.16, or 6283.2, and the horse-power would be 1/33, 000 of 6283.2, or about 19/100 of a horse-power. It will be seen, from this e ample, that if the resistance to motion in pounds and the speed of motion in feet per minute can be measured, it is only necessary to multiply them together, and divide by 33, 000, in order to obtain the horse-power. The steam-engine is a machine that is commonly rated as being of a certain horsepower, but the term horse-power, as thus used, does not always have the same meaning. In fact, there are four kinds of horse-power by which an engine may be rated:

1. Gross or indicated horse-power.

2. Net or effective

3. Total

4. Nominal

1. The gross or indicated horse-power of an engine is the power calculated by assuming the resistance to be that due to the mean effective pressure of the steam on the piston, as shown by the indicator. Thus, suppose this pressure is 2500 lbs., and the piston moves 400 ft. a minute, the gross horse-power is 400 times 2500 divided by 33, 000, or 30.3.

2. The net or effective horse-power of an engine is computed from the useful resistance overcome. If, in the preceding example, the pressure on the piston, after deducting that required to overcome the friction of the engine, is 3200 lbs., the effective horse-power is 2200 times 400, divided by 33, 000, or 26.7.

The net horse-power is the proper kind to be specified by a steam-user when he is buying an engine.

3. The total horse-power of an engine is computed from the total pressure on the piston above a vacuum. If, in the example given in Case 1, the total pressure on the piston is 4200 lbs., the total horsepower is 400 times 4200, divided by 33, 000, or 50.9.

Total horse-power is only used in comparisons of the results of experiments.

4. The nominal horse-power of an engine has no meaning in particular-that is to say, there are a number of different rules by which it may be computed. Thus, there is the admiralty rule for marine engines, Mr. Bourne's rule for condensing engines.

Mr. Bourne's rule for...... condensing engines, James Watt's rule; and numerous engine-builders have private rules of their own. For instance, .1 say-, "I will make an engine with a cylinder 10 ins. in diameter, and a stroke of 15 ins., and I will call it 8 horse power, nominal."

B', who builds an engine of the same size, and wants to make purchasers think they are getting more for their money, says, "I will call my engine 16 horse-power, nominal." The man who goes to buy a steam engine of either of these parties may very properly Bay to them, "How much will you charge me for an engine guaranteed to be of so many horse power, actual V" B. Indicator, The steam-engine, The construction is shown in Figs. 1 and 2, Fig. 1 being an elevation, and Fig. 2 a section.