88 THE POPULAR SCIENCE MONTHLY. 



during the first half of the British Association's life, been raised from 

 a mere formula of mathematical dynamics to the position it now holds 

 of a principle pervading all nature and guiding the investigator in 

 every field of science. 



A little article, communicated to the Royal Society of Edinburgh 

 a short time before the commencement of the epoch of energy, under 

 the title " On the Sources available to Man for the Production of 

 Mechanical Effect, 1 ' * contained the following : 



" Men can obtain mechanical effect for their own purposes by 

 working mechanically themselves, and directing other animals to work 

 for them, or by using natural heat, the gravitation of descending solid 

 masses, the natural motions of water and air, and the heat, or galvanic 

 currents, or other mechanical effects produced by chemical combina- 

 tion, but in no other way at present known. 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 electrically-deposited 

 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 

 term) is the principal source of mechanical effect available to man.f 

 From it is derived the whole mechanical effect obtained by means of 

 animals working, water-wheels worked by rivers, steam-engines, gal- 

 vanic engines, windmills, and the sails of ships. 



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

 attractions, constitute an important source of available mechanical 

 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. 



" 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 meteoric that is, be- 

 longing to bodies deposited on the earth from external space. Ter- 



* Read at the Royal Society of Edinburgh, on February 2, 1852 ("Proceedings" of 

 that date). 



f A general conclusion equivalent to this was published by Sir John Ilerschel in 1833. 

 See his "Astronomy," edition 1819, 399. 



