9.3 



at the point where constant frost prevails, i. e. the curve of con- 

 gelation. (See Plate.) The annexed table shows the relative 

 height of this curve in different latitudes from the level of the 

 sea: — 



atitude. 



Height of Curve 

 of Congelation. 



Mean temperature 

 at the level of the sea. 







16,000 



84-2 Fahr. 



10 



15,600 



82-6 



20 



14,500 



78-1 



30 



12,600 



71-1 



40 



10,000 



62-6 



50 



6,900 



53-6 



60 



3,900 



45*0 



70 



1,000 



38-1 



80 







32-0 



What we have stated above respecting the variable tempera- 

 ture of the sub-aerial zones according to the heights, we have 

 every reason to suppose is applicable to similar sub-aqueous 

 zones of variable temperature from the level of the sea down- 

 wards. At the equator, commencing from the level of the sea 

 upwards, we have the following : — 



Mean temperature. 

 Level of the sea 84°. 



4,000 feet high 70°, descending to the level of the sea in 30° lat. 



8,000 „ 62 „ „ „ „ 40 



12,000 „ 44 „ „ „ „ 60 



16,000 „ 32 „ „ „ „ 80 



From the level of the sea downwards the temperature decreases 

 in a similar manner, but the actual variations of temperature in 

 the sub-aqueous zones are not known. All that is positively 

 ascertained is, that within the tropics the ocean is found to be 

 getting colder as we descend. 



According to the concentrated rays, the heat of the sun ought 

 to increase in depth, and such would be the case in an empty 

 space; but liquids conduct heat very imperfectly downwards. 

 A thermometer let down a few feet below the surface of a pond 

 or of the sea, would, on being drawn up, indicate a lower tempe- 

 rature than that of the surface water ; for the latter, heated by 

 the rays of the sun, would communicate by conduction little or 



