It was not until 1755 that a mechanical means of producing low temperatures was developed. The inventor was Dr. Cullen, and he used an evaporation system, expediting the evaporation by producing a partial vacuum over the water. But nearly a century elapsed before the first commercially successful refrigerating machine was built. Even then the advantages of artificial refrigeration were not fully realized, and it was not until late in the last century that real progress was made. Since then the development of artificial refrigeration has been truly remarkable. HEAT AND MECHANICAL ENERGY There is a definite relation between heat and mechanical energy, in fact the two are mutually convertible. The amount of heat required to raise the temperature of a pound of water 1 degree F. is called a British thermal unit or a B. t. u. This measure is taken at 39.1 degrees F., because at that temperature water is at its densest. Since heat and mechanical energy are mutually convertible, we can express foot-pounds or horsepower in B. t. u. One B. t. u. is equivalent to 778 foot-pounds of energy. In other words, the amount of heat that would raise the temperature of a pound of water 1 degree F. would, if converted into mechanical energy, be sufficient to raise a weight of 778 pounds to a height of one foot, or one pound to a height of 778 feet. A horsepower is equivalent to 2,545 B. t. u. per hour. Heat from burning coal is used to generate steam, and this in turn is used to operate a steam engine and thus heat is converted into mechanical energy (unfortunately most of the original heat units in the coal are wasted, as was pointed out in a previous chapter); but heat will not flow from one body into another of higher temperature without the expenditure of mechanical energy. It always flows from a hot body into a cold one, and not from the cold body into the hot one, unless it is actually pumped up to the higher heat level by some mechanical means. A refrigerating machine is actually a heat pump with which we produce a partial heat vacuum. Whenever a gas is compressed, heat is generated. Anyone who has operated a tire pump knows how hot the pump becomes from the heat that is seemingly squeezed out of the compressed air. As was noted in Chapter VIII, heat is liable to give trouble in an air compressor, and sometimes the temperature rises to such a point that there is an explosion of the air and the vapors coming from the oil used to lubricate the machine. The compressed air is therefore cooled by means of water jackets or coils of pipe through which water is passed. In this way the excess heat is carried off. When, however, cooled compressed air is relieved of pressure and allowed to expand again the process is reversed. A partial heat vacuum is formed and heat from surrounding objects flows into the vacuum. In other words, the surrounding objects are cooled. COLD AIR MACHINES [Illustration: FIG. 79.--COLD AIR MACHINE]