SECTION V. HEAT, WARMING, AND VENTILATING; MELT. ING, CUTTING, AND FINISHING METALS; ALLOYS AND CASTING; WHEEL-CUTTING; SCREW-CUTTING, &c. HEAT. Unit of Heat.-The amount of heat necessary to raise the temperature of one pound of water at 32° one degree Fahrenheit (that is from 32° to 33°) is called the standard unit of heat. Table 45-SPECIFIC HEAT OF SOLID AND LIQUID BODIES, BEING THE FRACTION OF A UNIT OF HEAT NECESSARY TO HEAT ONE POUND OF THE BODY ONE DEGREE FAHRENHEIT. FROM THE EXPERIMENTS OF REGNAULT AND DULONG. The Specific Heat of a body is its power of storing up heat, and the number of units of heat necessary to heat one pound of the body 1° Fahr. is its specific heat, water being used as the standard of comparison. Thus, to heat 100 lbs. of water 80° requires 100 × 80 = 8000 units of heat, and to heat the same weight of wrought-iron requires 100 x 80 x 113 = 904, or only about th of the heat necessary for the same weight of water. Table 46.-EXPANSION OF LIQUIDS AND GASES IN VOLUME BY THE ADDITION OF HEAT FROM 32° TO 212° F. Table 47.-HEAT-CONDUCTING POWER OF METALS, &c.—Latent HEAT. Table 48.-EXPANSION IN LENGTH OF METALS, &C., BY HEAT PER DEGREE FAHRENHEIT FROM 32°. Table 49.-RADIATION, ABSORPTION AND REFLECTION OF HEAT. Superficial expansion or expansion in two directions, is twice the linear. expansion; and cubical expansion, or expansion in three directions, is three times the linear expansion. The Quantity of Heat given in Table 50 for each material named, is deduced from experiments on the transmission of heat through plates of metal, which were heated on one side by hot water, and cooled on the other side by water at a low temperature. The quantity of heat in units, transmitted through one square foot of plate, per hour, may be found thus: subtract the temperature of the cooler side, from that of the hotter side of the plate, then multiply the result by the number in Table 50 corresponding to the material used, and divide the product by the thickness of plate. Thus an iron plate 2 inches thick, having a temperature of 60° on one side 20 X 230 and 80° on the other, will transmit 80-60 = heat, per square foot per hour. 2 = 2300 units of Table 50.-QUANTITY OF HEAT IN UNITS TRANSMITTED PER SQUARE foor PER HOUR, THROUGH A PLATE I INCH THICK, THE DIFFERENCE OF TEMPERATURE BETWEEN THE TWO FACES BEING 1° F.-FROM THE EXPERIMENTS OF PECLET. HEATING ROOMS BY HOT WATER. A Hot Water Boiler with its flow and return pipe, resembles an inverted syphon; the motive power in the circulation of hot water, is the difference in weight between the columns of water, ascending from the boiler through its top outlet, or flow pipe, and returning to the boiler through its bottom inlet, or return pipe. As the water in the boiler is heated it expands, becomes lighter and ascends to the top of the boiler in the direction of the flow pipe, and is replaced by colder and consequently heavier water from the bottom or return pipe; this in turn gets heated, ascends, and is replaced by more cold water from the return pipe, and this circulation continues so long as the fire is kept up, the hot water continually ascending, and the cold water descending. Mr. Hood, who is an authority on this subject, gives the following tables for heating rooms by hot water. |