Page:Physical Geography of the Sea and its Meteorology.djvu/247

Rh temperature dips again, and continues to incline till it reaches the poles at the depth of 750 fathoms. So that on the equatorial side of the outcrop the water above this floor is the warmer, but on the polar side the supernatant water is the colder. By this floor with its waters of one uniform and permanent temperature, "the ocean," says Sir John Herschel, "is divided into three great regions—two polar basins in which the surface temperature is below, and one medial zone in which it is above 39°. 5, being 80° at the equator; and at the poles, of course, the freezing-point of sea water. It will be very readily understood that in this statement there is nothing repugnant to hydrostatical laws, the compressibility of water insuring an increase of density in descending within much wider limits of temperature than here contemplated."

441. Thermal dilatation of the water.221 The temperature of 39°.5 was assigned to this floor probably under the supposition that sea water follows fresh in its laws of thermal dilatation. Not so; while fresh water attains its maximum density at 39°.5, average sea water does not arrive at its degree of maximum density until it passes its freezing-point (27°.2) and reaches the temperature of 25°.6. In the winter of 1858 a very elaborate series of observations was conducted at the National Observatory, by Professor Hubbard, upon the thermal dilatation of sea water, and with the following results, 60° being the standard temperature :

Thermal Dilatation of Sea Water