TERRESTRIAL HEAT. 



greater, a fact which, is in accordance with what has been already 

 explained. 



It has not been certainly ascertained whether the hot springs, 

 some of which rise to a temperature little less than that of 

 boiling water, derive their heat from the great depth of the strata 

 from which they rise, or from local conditions affecting the strata. 

 The uniformity of the temperature of many of them appears to 

 favour the former hypothesis ; but it must not be forgotten that 

 other geological conditions besides mere depth may operate with 

 the same permanency and regularity. 



30. It is well known that bodies in general expand when they 

 are heated, and contract when they are c*ooled. Water, when its 

 temperature falls below 40, presents a most remarkable exception 

 to this general law. It continues in accordance with the law to 

 contract, though in a continually diminished degree, from 40 to 

 38 *8, and when it arrives at the latter temperature the contraction 

 ceases altogether. "When its temperature falls below 3S 0t S, instead 

 of contracting, it expands, and it continues to expand until it is 

 frozen, which takes place at 32. 



It follows from this that the density of water, or its weight 

 bulk for bulk compared with itself at different temperatures is 

 greatest when it has the temperature of 38 0< 8, which is therefore 

 called the temperature of greatest density.* 



31. This anomalous quality of water when its temperature falls 

 below 38 '8 Fahr. and its consequent maximum density at that 

 temperature, is attended with most remarkable and important 

 consequences in the phenomena of the waters of the globe, and in 

 the economy of the tribes of organised creatures which inhabit 

 them. It is easy. to show that, but for this provision, exceptional 

 as it feeems, disturbances would take place, and changes ensue, 

 which would be attended with effects of the most injurious 

 description in the economy of nature. 



If a large collection of water, such as an ocean, a sea, or a lake, 

 be exposed to continued cold, so that its superficial stratum shall 

 have its temperature constantly reduced, the following effects will 

 be manifested. 



The superficial stratum falling in temperature, will become 

 heavier, volume for volume, than the strata below it, and will 

 therefore sink, the inferior strata rising and taking its place. 

 These in their turn being cooled will sink, and in this manner 

 a continual system of downward and upward currents will be 

 maintained, by means of which the temperature of the entire mass 

 of liquid will be continually equalised and rendered uniform from 



* See Tract on Water (5). 

 74 



