TERKEST111AL HEAT. 



freezing point, increases in descending until it rises to the freezing 

 point, at the stratum where the ice ceases, and the liquid water 

 commences. Below this the temperature still augments until it 

 reaches 3S'8, the temperature of maximum density of water, and 

 this temperature is continued uniform to the bottom. 



33. Let us now consider what effects will be produced, if the 

 superficial strata be exposed to an increase of temperature. After 

 the fusion of the ice, the temperature of the surface will gradually 

 rise from 32 to 38-8, the temperature of greatest density. When 

 the superficial stratum rises above 32, it will become heavier 

 than the stratum under it, and an interchange by currents, and a 

 consequent equalisation of temperature, will take place, and this 

 will continue until the superficial stratum attain the temperature 

 of 38 -8, when the temperature of the whole mass of water from 

 the surface to the bottom will become uniform. 



After this a further elevation of the temperature of the super- 

 ficial stratum will render it lighter than those below it, and no 

 currents will be produced, the liquid remaining at rest ; and this 

 state of repose will continue so long as the temperature continues 

 to rise. 



Every fall of the superficial temperature, so long as it con- 

 tinues above 38'8, will be attended with an interchange of 

 currents between the superficial and those inferior strata whose 

 temperature is above 38*8, and a consequent equalisation of 

 temperature. 



34. It appears, therefore, to result as a necessary consequence 

 from what has been explained, and this inference is fully confirmed 

 by experiment and observations, that there exists in oceans, seas, 

 and other large and deep collections of water, a certain stratum, 

 which retains permanently, and without the slightest variation, 

 the temperature of 38 '8, which characterises the state of greatest 

 density, and that all the inferior strata equally share this tempera- 

 ture. At the lower latitudes, the superior strata have a higher, 

 at the higher latitudes a lower temperature, and at a certain mean 

 latitude the stratum of invariable temperature coincides with the 

 surface. 



In accordance with this, it has been found by observation that 

 in the torrid zone, where the superficial temperature of the sea is 

 about 83, the temperature decreases with the depth until we 

 attain the stratum of invariable temperature, the depth of which, 

 upon the Line, is estimated at about 7000 feet. The depth of this 

 stratum gradually diminishes as the latitude increases, and the 

 limit at which it coincides with the surface is somewhere between 

 53 and 60. Above this the temperature of the sea increases as 

 the depth of the stratum increases, until we sink to the stratum 

 7G 



