PRODUCTION OF MECHANICAL EFFECT. 435 



" I. The food of animals. 



" II. Natural heat. 



"III. Solid matter found in elevated posi- 

 " tions. 



" IV. The natural motions of water and air. 



C 'V. Natural combustibles (as wood, coal 

 " coal-gas, oils, marsh-gas diamond, native sul- 

 " phur, native metals, meteoric iron). 



" VI. Artificial combustibles (as smelted or 

 " electrically-deposited metals, hydrogen, phos- 

 " phorus). 



" 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 con- 

 " elusions : 



"i. Heat radiated from the sun (sunlight being 

 " included in this term) is the principle source of 

 " mechanical effect available to man. 1 From it is 

 " derived the whole mechanical effect obtained by 



1 A general conclusion equivalent to this was published by Sir 

 John Ilerschel in 1833. See his Astronomy, edit. 1849, (399). 



