32 Voyage of Capt. Sir James C. Ross to the Antarctic. 



mediate latitudes, giving a mean latitude of 56° 14/. " It is 

 evident therefore 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 between 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 temperature at various depths 

 in different latitudes. So that the line of mean temperature of 

 39°'5, in lat. 45° S., has descended to the depth of 600 fathoms; 

 and at the equatorial and tropical regions, 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 temperature of 

 39°-5, whilst that of the surface is 78°." " So likewise to the 

 south of the circle of mean temperature, we find that in the ab- 

 sence 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, 

 we 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 0, 5 obtains whilst that of the surface is 30°." 



" This circle of mean temperature of the southern ocean is a 

 standard 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 temperature, and to what amount during the interval. 77 



" These observations force upon us the conclusion that the 

 internal heat of the earth exercises no influence upon the temper- 

 ature of the ocean, or we should not find any part iu 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." 



April 4. — They arrived at the Cape of Good Hope, returning 

 for the third time from the Antarctic circle "without a single in- 

 dividual of either of the ships upon the sick list." 



April b0. — They left the Cape, touching at St. Helena and the 

 Island of Ascension, and at ail these places as well as at Rio 

 Janeiro, the magnetic experiments were repeated to their satis- 

 faction.' On the 3d of June, in lat. 15° 3' S., long. 23° 14' W., 

 they failed to obtain soundings at the depth of 4600 fathoms, or 

 5£ miles, the greatest depth of water in the ocean that had been 

 actually ascertained ; but it is not improbable that there are 

 depths still more profound. 



July 3. — They crossed the line of no dip, in lat. 13° 20' S., long. 

 28° 11' W., and of course were on the magnetic equator. Their 

 " barometrical experiments had proved that the atmospheric pres- 

 sure is considerably less at the equator than near the tropics, and 

 south of the tropic of Capricorn, where it is the greatest, a gradual 



