330 THE POPULAR SCIENCE MONTHLY. 



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 compact- 

 ness over the steam-engine and boiler. This greater size of the work- 

 ing parts also increases friction largely, so much indeed that what- 

 ever 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 service- 

 able 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 broadlj'^ divided into two classes by the manner in which 

 they use the air. In one class it is drawn directly from the atmos- 

 phere, 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 pres- 

 sure, 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- 



