Assuming that the earth once formed part of the sun, the whole of the energy at our command for commercial purposes can be traced back to the sun as source. This energy we have received from it in the form of heat, and under certain circumstances the heat is stored up in a latent form in chemical compounds such as coal, petroleum, etc. With our present knowledge it is exceedingly difficult to extract the latent energy from coal and petroleum in any other form but heat, and in order to do so to our greater benefit, it is necessary to study the laws of heat and heat engines. The law which states the relation between heat and other forms of energy such as electricity, mechanical work, is called the principle of the conservation of energy, and forms the first law of thermodynamics. It is enunciated as follows. Whenever a body does work or has work done upon it, there is a disappearance or an appearance of heat, and the amount of heat thus produced or used up is always exactly proportional to the work which is done. The ratio of the amount of work which a certain quantity of heat can produce has been therefore termed the mechanical equivalent of heat.
It has been found by experiment, taking the calorie (C.G.S. unit) as the unit of heat and the kilogramme metre as the unit of work or energy, that the mechanical equivalent is 424. That is to say, the heat necessary to raise the temperature of one kilogramme of pure water at 0° Centigrade through 1° C. (the calorie) is equal to the work done in raising 424 kilogrammes to a height of one metre.
In nearly all commercial heat engines the heat is converted into the energy of movement (kinetic energy) by using some body such as water vapour, gas, or air as an intermediary agent. We do not, however, know at present how to transform heat into mechanical work without losing a greater part of it in the process. Even in the most perfect heat engines at least 70% of the heat is lost, only about 30% being converted into mechanical energy. This is as yet the most perfect result which engineers have obtained even with the most elaborate precautions. As a rule the loss is greater; for instance, many good machines which we consider efficient burn one kilogramme of coal, giving out 8000 calories, equivalent to 3,400,000 kilogramme-metres, and transform only about 400,000 kilogramme-metres into work, the rest, forming nearly 80%, is lost.