30 



TEMPERATURE OF THE OCEAN. 



greatest density, has a uniform temperature of about 

 39 5' Fahr. ; and that when heated above, or cooled 

 below this temperature it becomes specifically lighter. 

 Hence we should expect that, in every column of 

 water sufficiently deep, a portion of the column will 

 be of this temperature. Now it has been tested 

 by experiments that in all latitudes the tempera- 

 ture of the ocean beyond a certain depth, which 

 varies with the latitude, has this uniform temperature 

 of 39 5'. Under the equator the temperature gra- 

 dually diminishes from the surface as the depth in- 

 creases, until at about 7200 feet the water is cooled to 

 39 5', which is the temperature of all greater depths. 

 Receding from the equator, we reach the uniform tem- 

 perature of 39 5' at a less and less depth, until at 

 about 56 north and south latitude, we find the waters 

 of the surface, and all below them, cooled down to 

 this uniform standard. Up to lat. 56 the superficial 

 strata are warmer than the inferior ones; but in all 

 higher latitudes they are colder, the plane of 39 5' 

 FIG. 1. Fahr. gradually sinking deeper 



and deeper, until at about the 

 latitude of 80 it is depressed 

 4500 feet below the surface of 

 the sea. This may be rendered 

 evident by a simple figure (fig. 

 1), where the circle represents 

 the surface of the sea, and the 

 curved figure enclosed what 

 we may call the plane of uni- 

 form temperature, which is depressed towards the equa- 



