Page:A Voyage of Discovery and Research in the Southern and Antarctic Regions Vol 2.djvu/422

378 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°.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.

The experiments which our limited time and means admitted of our making, serve to show that the mean temperature of the ocean at present is about 39°.5, or 7½ degrees 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 ascertained to the tenth part of a degree; and as we now know where we may send any number of thermometers down to the greatest fathomable 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