This short history of the gas engine will be seen to consist of three distinct periods—firstly, from 1700 up to 1860, during which time many inventors tried and failed to produce anything practical; secondly, from 1860 to 1889, during which the gas engine became something really practical; thirdly, from 1889 up to the present date. In this period gas engines have grown in size, and large units of 200 to 400 horse-power are now constructed, worked by poor gas produced from special gas plants, and enabling the gas engine to successfully hold its own against the steam engine, which it may one day entirely supplant. CHAPTER II THE WORKING PRINCIPLES OF THE GAS ENGINE 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.