Prime Movers. 13 



The force of the wind was applied very early to the propul- 

 sion of ships ; but it was not employed to drive machinery until 

 a comparatively recent period. Wind-mills are said, by some, to 

 have been invented in France, in the sixth century; it has been 

 asserted by others that they were known in Greece and Arabia 

 in the seventh century, but were first introduced into these 

 countries during the Crusades. They have been very com- 

 monly used, particularly in Holland, for drainage; but theii 

 frequent stoppage for want of wind, and the difficulty of regu- 

 lating their speed, has been a serious obstacle to their adoption 

 in most manufactures. — The force obtained from the wind, also, 

 is derived from the sun : which, by rarefying the air in distant 

 regions, causes atmospheric currents to be produced. 



Before entering on the subject of steam-engines, and their 

 proposed substitutes, it will be useful to glance at a few of the 

 properties of Heat, which, with the one exception we have 

 mentioned, is probably the source of all our motive power. 

 Heat is employed either to cause an increase of temperature, in 

 which case it is said to be sensible ; or to produce mechanical 

 changes, in which case it is termed latent. The work which is 

 lost by friction is always expended in the development of heat ; 

 and hence it is supposed, that the friction of a piston against 

 the sides of a steam cylinder causes no diminution in the power 

 of the engine, since the heat which is set free raises the tem- 

 perature of the steam, and therefore augments its pressure. 

 The effect producible by 772 pounds falling one foot, is con- 

 sidered to be the quantity of work, corresponding with the heat, 

 which would raise a pound of water, at the ordinary temperatures, 

 through one degree Fahrenheit ; and this quantity has been 

 termed " Joule's unit." When a body returns to its original 

 condition — steam, for example, to water — heat disappears, and 

 to an extent equal to that which would be generated by em- 

 ploying the force thus produced in overcoming friction. This 

 is called "the conversion of heat into mechanical energy." And 

 the efficiency of the heat, in a heat-engine, depends on the 

 relation between the heat converted into mechanical energy, 

 and the whole heat applied. It is probable that the efficacy 

 of all prime movers, without exception, is proportional to the 

 heat so converted. The efficiency of a steam-engine depends 

 on that of the furnace, or the effect of the steam upon the piston, 

 and on the power communicated by the piston, through the 

 crank shaft, to the machinery ; but nearly one-fifth of the heat 

 is expended in causing a draught in the chimney, and a large 

 quantity besides is lost by radiation from the various parts. 



There is great reason to believe, that all sources of power 

 yield the same amount of it, with the same quantity of heat ; 

 and that the only thing we can do is to discover in what 



