The term horsepower, when applied to the boiler, has a meaning scarcely more definite than when used to indicate the capacity of the engine. In either case, the horse power realized depends as much upon the method of using the mechanism, as upon its original construction. The best authorities' agree that the horse-power of the boiler should indicate the actual evaporation of water, instead of the size of the boiler or the efficiency which may be secured through the engine: The ability to evaporate a cubic 'foot of water per boor, making steam at 212° F., has been suggested as a suitable unit to be called a horse-power. To ascertain the evaporative power of a boiler by experi ment, it is necessary to obtain the weights of fuel and water, and to know the quality of the steam produced. A trial should last 24 hours; steam may be raised, and then •fire withdrawn, and the ash-pit Cleared, the steam meanwhile being maintained with wood. Coal is then added, and as soon as it is fired, the test begins. Note is taken of the height of water In the gauge, and the water is left at the same height at the end of the test. Coal is carefully weighed in regular amounts and at. regular intervals to avoid, errors. At the end of the trial the fire•is withdrawn, and the remaining coal weighed as soon as possible; this weight, plus that of the ashes made during the experiment, taken from the weight of the coal. gives the weight of fuel consumed. To find the quality of the steam, a tank is provided, which is traversed by a pipe leading to the boiler, the whole apparatus being so arranged tis to waste as•little heat as possible. The. tank is filled with water, and steam is admitted through the pipe in such quantity ns may be condensed by the water. We have to note the pressure of the steam, the weight and temperature of the water before steam is admitted, the weight and temperature at the close of the test, the weight and temperature of the water formed from the condensed steam,.and the time. Experiment must also b® made to test the loss of heat by radiation and evapo ration, which is done by heating a given quantity of water to a given temperature in the same tank, and noting the loss in weight and temperature during a given time. To illus trate by an example. Suppose a test made, from which these data have been secured: Coal used, 5980 lbs.; feed water used, 42,320 lbs.; coal withdrawn at end of test, with
ashes, 1830 lbs.; hence, coal Unmet] in the test, 5980 minus 1830=41501bs. The apparent evaporation per pound of coal is, 42,820+-4150=10.2 lbs., if the steam were dry. To test the quality of the steam the described apparatus has been used, and these data uoted: Pressure of steam at gauge, 80 lbs.; weight of steam condensed at 05', 204 lbs.; initial temperature of water for condensing, 60'; final temperature, 92'; head of water in tank, 27 in. ; time of trial, 24 hours; and by former tests it appears that 4 cu. ft. of water, weighing 62.2 lbs. per ft., pass from Ilia tank per hour, and that the loss of heat by evaporation and radiation is 1480 thermal units per hour. The heatgiven to the water by the condensing steam in one hour was 4X 62.2 X (95 minus 60) +1480=8708 thermal units. The steam condensed per hour was 204+24=8.5 lbs., hence each pound of steam com municated to the water 8708+8.5=1024.5 thermal units of heat. But this condensed steam was discharged at 05'; to bring it down to the standard of 32° there must have been a farther reduction of 95 minus 30=65 thermal units, showing that the quantity of heat above freezing standard held by a pound Of steam as it issued from the boiler was 1024.5+65=1089.5 thermal units. The total heat, above freezing standard, of a pound of dry steam at 80 lbs. pressure (see Rankine, ,Stearn Ellgtile, or Appleton's Cyr. of Mechanics), is 1177.1; it is therefore evident that the steam used in the. test contained some moisture. As the temperature of the feed-water was 60', it had already 28 thermal units of heat per pound above water at 32', and would require 1177.1 minus 28=1140.1 thermal units to change it to dry steam; but it required 1089.5 minus 28=1001.5 thermal units to change it to steam of the quality observed, hence the actual evaporation was 1061.54-1140.1=0.01506 of the apparent evaporation. But the apparent evaporation was 8.5 lbs. per pound of coal, and the actual was therefore 7.778 lbs. If the feed-water were at 212', 908.5 thermal units would be required to convert a pound of water into steam. Hence, 1061.5÷908.5=10.6= nearly tho evaporation per pound from and ut 212'.