436 POPULAR LECTURES AND ADDRESS1-.S. 



" means of animals working, water-wheels worked 

 " by rivers, steam-engines, galvanic engines,, wind- 

 " mills, and the sails of ships. 



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

 " and their mutual attractions, constitute an im- 

 " portant source of available mechanical effect. 

 " From them all, but chiefly no doubt from the 

 " earth's motion of rotation, is derived the ine- 

 ' chanical effect of water-wheels driven by the 

 " tides. 



" 3. The other known sources of mechanical 

 " effect available to man are cither terrestrial 

 " that is, belonging to the earth, and available 

 " without the influence of any external body or 

 " meteoric 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 inorganic native combustibles, are actually 

 " used ; but the mechanical effect obtained from 

 ." them is very inconsiderable, compared with 

 " that which is obtained from sources belonging 



