Page:A Voyage of Discovery and Research in the Southern and Antarctic Regions Vol 2.djvu/419

] We had previously crossed this circle at the following six different points, viz.:—

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, 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 latitude 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 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