THEORY of the EARTH. 213 



produced the viciffitudes of night and day, fo variable in the 

 different latitudes from the equator to the pole, and fo beauti- 

 fully calculated to equalize the benefits of light, fo varioufly 

 diftributed in the different regions of the globe. 



GRAVITATION and the vis incita of matter thus form the 

 firft two powers diftinguifhable in the operations of our fyftem r 

 and wifely adapted to the purpofe for which they are employed. 



WE next obferve the influence of light and heat, of cold and 

 condenfation. It is by means of thefe two powers that the va- 

 rious operations of this living world are -more immediately 

 tranfacted ; although the other powers are no lefs required, in 

 order to produce or modify thefe great agents in the ceconomy 

 of life, and fyftem of our changing things. 



WE do not now enquire into the nature of thofe powers, or 

 invefligate the laws of light and heat, of cold and eondenfa- 

 tion, by which the various purpofes of this world are accom- 

 plifhed; we are only to mention thofe effects which are made 

 fenfible ta the common underftanding of mankind, and which 

 neceffarily imply a power that is employed. Thus, it is by the 

 operation of thofe powers that the varieties of feafon in fpring 

 and autumn are obtained, that we are bleffed with the viciffi- 

 tudes of fummer's heat and winter's cold, and that we poffefs 

 the benefit of artificial light and culinary fire. 



WE are thus bountifully provided with the neceflaries of 

 life ; we are fupplied with things conducive to' the growth and 

 prefervation of our animal nature, and with fit fubjects to em- 

 ploy and to nourifh our intellectual powers. 



THERE are other actuating powers employed in the opera- 

 tions of this globe, which we are little more than able to enu- 

 merate J fuch are thofe of electricity and magnetifm. 



POWERS of fuch magnitude or force, are not to be fuppofed 

 nfelefs in a machine contrived furely not without wifHom ; but 

 they are mentioned here chiefly on account of their general 

 effect , and it is fufficient to have named powers, of which the 



actual 



