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CHAPTER IX. 



The Laws of Heat with respect to Water. 



THE manner in which heat is transmitted through 

 fluids is altogether different from the mode in 

 which it passes through solids ; and hence the 

 waters of the earth's surface produce peculiar 

 effects upon its condition as to temperature. 

 Moreover, water is susceptible of evaporation in 

 a degree depending upon the increase of heat ; 

 and in consequence of this property it has most 

 extensive and important functions to discharge 

 in the economy of nature. We will consider 

 some of the offices of this fluid. 



1. Heat is communicated through water, not 

 by being conducted from one part of the fluid to 

 another, as in solid bodies, but (at least princi- 

 pally) by being carried with the parts of the fluid 

 by means of an intestine motion. Water expands 

 and becomes lighter by heat, and, therefore, if the 

 upper parts be cooled below the subjacent tem- 

 perature, this upper portion will become heavier 

 than that below, bulk for bulk, and will descend 

 through it, while the lower portion rises to take 

 the upper place. In this manner the colder parts 

 descend, and the warmer parts ascend by con- 

 trary currents, and by their interchange and 



