ITS DIVISIONS MUST ADVANCE TOGETHER. 185 



action ; that the ability of any portion of matter to mani 

 fest visible gravitative phenomena depends on its state of 

 aggregation, which is determined by heat ; that only by 

 the aid of thermology can we explain those apparent ex 

 ceptions to the gravitating tendency which are presented 

 by steam and srnoke, and so establish its universality, and 

 that, indeed, the very existence of the solar system in a sol 

 id form is just as much a question of heat as it is one of 

 gravitation. 



Take other cases : All phenomena recognised by tho 

 eyes, through which only are the data of exact science as- 

 ccrtainable, are complicated with optical phenomena ; and 

 cannot be exhaustively known until optical principles are 

 known. The burning of a candle cannot be explained 

 without involving chemistry, mechanics, thermology. 

 Every wind that blows is determined by influences partly 

 solar, partly lunar, partly hygrometric ; and implies con 

 siderations of fluid equilibrium, and physical geography 

 The direction, dip, and variations of the magnetic needle, 

 are facts half terrestrial, half celestial are caused by earth 

 ly forces which have cycles of change corresponding with 

 astronomical periods. The flowing of the gulf-stream and the 

 annual migration of icebergs towards the equator, depend 

 ing as they do on the balancing of the centripetal and centri 

 fugal forces acting on the ocean, involve in their explana 

 tion the Earth s rotation and spheroidal form, the laws of 

 hydrostatics, the relative densities of cold and warm water, 

 and the doctrines of evaporation. It is no doubt true, aa 

 M. Comte says, that &quot; our position in the solar system, and 

 the motions, form, size, equilibrium of the mass of our 

 world among the planets, must be known before we can un 

 derstand the phenomena going on at its surface.&quot; But, fa 

 tally for his hypothesis, it is also true that we must under 

 stand a great part of the phenomena going on at its surface 

 before we can know its position, &c., in the solar system 



