gas at two dollars per thousand, would only cost a little more than two cents an hour, which would make the expense of running a sewing-machine as commonly used inconsiderable. The first cost of the motor is low, while its simplicity of construction should render it quite durable.
The advantages possessed by engines driven by heated air over those using steam, in the matter of safety, absence of the need of skilled attendance, and a possible higher efficiency, early turned the attention of inventors toward the development of this class of motors. Promising as the field seemed, the practical realization of the hopes indulged in has been far short of expectation. The rapid alternate heating and cooling of the air, and the prevention of excessive wear due to traveling surfaces becoming highly heated, have been found to be difficult things to accomplish satisfactorily. On account of the low pressure obtained from heated air, engines of this kind have to be made of such large dimensions that there is little if any gain in compactness over the steam-engine and boiler. This greater size of the working parts also increases friction largely, so much indeed that whatever superior efficiency the hot-air engine may possess, from being able to work between greater extremes of temperature, disappears in the power required to move simply the machine itself. In large powers this engine has never yet been able to compete with the steam-engine, but in small ones it has proved to be quite an economical and serviceable motor. A number of engines of this kind have been invented at different times, and some have gone more or less largely into use. The earliest to excite interest, and to come into a limited use, was that of Rev. Dr. Stirling, in 1816, which was successively improved by him and his brother up to 1840. Among the engines which have been more or less successful have been those of Ericsson, Wilcox, Roper, Shaw, and Rider in this country, and Lauberau and Belou in France. Hot-air engines are broadly divided into two classes by the manner in which they use the air. In one class it is drawn directly from the atmosphere, used, and then discharged. In the other the same body of air is used continuously, being alternately heated and cooled. The latter class has the advantage of being able to use the air at a greater pressure, but they need a refrigerating apparatus, which is unnecessary with the first. When the engine is used for pumping purposes this constitutes no disadvantage, as the water can be passed around the part of the cylinder desired to be kept cool, and even when it is used as a motor, a water circulation can very readily be kept up by means of a supply tank of sufficient capacity. As the efficiency of any heat engine depends upon the extremes of temperature between which it is worked, either the air used in the air-engine should leave the power cylinder at the temperature of the atmosphere, or it must be made to give up a portion of its heat to some apparatus that will yield it again to the entering air. The first is impracticable, and attention has there-