260 Prof. Thomson on the Production of Mechanical Effect. 



and the heat, or galvanic currentSj or other mechanical effects 

 produced by chemical combination, but in no other way at pre- 

 sent knowai. Hence the stores from which mechanical effect may 

 be drawn by man belong to one or other of the following 

 classes : — 

 I. The food of animals. 

 II. Natural heat. 



III. Solid matter found in elevated positions. 



IV. The natural motions of water and air. 



V. Natural combustibles (as wood, coal, coal-gas, oils, marsh 

 gas, diamond, native sulphur, native metals, meteoric iron). 

 VI. Artificial combustibles (as smelted or electrolytically depo- 

 sited metals, hydrogen, phosphorus). 

 In the present communication, known facts in natural history 

 and physical science, with reference to the sources from which 

 these stores have derived their mechanical energies, are adduced 

 to establish the following general conclusions : — 



1. Heat radiated from the sun (sunlight being included in this 

 tei'm) is the pj-incipal source of mechanical effect available to man^. 

 From it is derived the whole mechanical effect obtained by means 

 of animals working, water-wheels worked by rivers, steam- 

 engines, and galvanic engines, and part at least of the mechanical 

 effect obtained by means of windmills and the sails of ships not 

 driven by the trade-winds. 



2. The motions of the earth, moon, and sun, and their mutual 

 attractions, constitute an important source of available mecha- 

 nical effect. From them all, but chiefly, no doubt, from the 

 earth's motion of rotation, is derived the mechanical effect of 

 water-wheels driven by the tides. The mechanical effect so 

 largely used in the sailing of ships by the trade-winds is derived 

 partly, perhaps principally, from the earth's motion of rotation, 

 and partly fi-om solar heat. 



3. The other known sources of mechanical effect available to 

 man are either terrestrial — that is, belonging to the earth, and 

 available without the influence of any external body, — or me- 

 teoric,— that is, belonging to bodies deposited on the earth from 

 external space. Terrestrial sources, including mountain quarries 

 and mines, the heat of hot springs, and the combustion of native 

 sulphur, perhaps also the combustion of all inorganic native com- 

 bustibles, are actually used, but the mechanical eftect obtained 

 from them is very inconsiderable, compared with that which is ob- 

 tained from sources belonging to the two classes mentioned above. 

 Meteoric sources, including only the heat of newly-fallen meteoric 

 bodies, and the combustion of meteoric iron, need not be rec- 

 koned among those available to man for practical purposes. 



* A general conclusion equivalent to this was published bj' Sir John 

 Herschelin 1833. See his Astronomy, edit. 1849, ^ (399). 



