402 Scientific Intelligence-^Hydrography. 



HYDROGRAPHY. 



9. Temperature of the Ocean. — — It is, therefore, evident that 

 about this parallel of latitude, there is a belt or circle round the 

 earth, where the mean temperature of the sea obtains throughout 

 its entire depth, forming a boundary, or kind of neutral ground, be- 

 tween the two great thermic basins of the ocean. To the north of 

 this circle the sea has become warmer than its mean temperature 

 by reason of the sun's heat, which it has absorbed, elevating its tem- 

 perature at various depths in different latitudes. So that the lino 

 of mean temperature of 39°* 5, in latitude 45° S., has descended to 

 the depth of 600 fathoms ; and at the equatorial and tropical re- 

 gions, this mark of the limit of the sun's influence is found at the 

 depth of about 1200 fathoms ; beneath which the ocean maintains 

 its unvarying mean temperature of 39°* 5, whilst that of the surface 

 is about 78°. 



So likewise to the south of the circle of mean temperature, we 

 find that in the absence of an equal solar supply, the radiation of 

 the heat of the ocean into space occasions the sea to be of a colder 

 temperature as we advance to the south ; and near the 70th degree 

 of latitude, v/e find the line of mean temperature has descended to 

 the depth of 750 fathoms; beneath which, again, to the greatest 

 depths, the temperature of 39°'5 obtains, whilst that of the surface 

 is 30°. 



This circle of mean temperature of the Southern Ocean is a stand- 

 ard point in nature, which, if determined with very great accuracy, 

 would afford to philosophers of future ages the means of ascertaining 

 if the globe we inhabit shall have undergone any change of tem- 

 perature, and to what amount, during the interval. 



The experiments which our limited time and means admitted of 

 our making, serve to shew that the mean temperature of the ocean 

 at present is about 39°*5, or 1^° above the freezing point of pure 

 water ; and as nearly as possible the point of its greatest density. 

 But it would be indispensable that this temperature should be as- 

 certained to the tenth part of a degree ; and as we now know where 

 we may send any number of thermometers down to the greatest fa- 

 thomable depths, without an alteration of temperature, even to that 

 small amount, this desideratum might be very easily obtained. 



These observations force upon us the conclusion, that the internal 

 heat of the earth exercises no influence upon the temperature of the 

 ocean, or we should not find any part in which it was equable from the 

 surface to the great depth we have reached ; a new and important 

 fact in the physics of our globe. — Sir James C* Mosses Voyage to the 

 Southern Seas, vol. ii., p. 377. 



10. Low Temperature of the Sea over Sand-Banks.^' — *' As no 

 sea can be supposed absolutely motionless, the presence of a shoal, 

 by casting up to the surface water which, but for it, would have con- 



* Sir James C. Ross's Voyage to the Southern Seas, vol. ii., p. 35. 



