THE SPEOIFIC GRAVITY OF THE SEA, ETC. 



221 



temperature tlips 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 Ilerschel, "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 state- 

 ment there is nothing repugnant to hydrostatical laws, the com- 

 pressibility of water insuring an increase of density in de- 

 scending within much wider limits of temperature than here 

 contemplated." 



441. Thermal dilatation of the water. — 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; 

 Avhile 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°.G. In the winter of 1858 a very elaborate series of observa- 

 tions 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 TFato-.f 



='•= Tliid remark was made by Sir Jolin on the supposition, probably, that tlie 

 maximum density of sea water was at the same temperature as that of fresh, but 

 it is the same 12^ or 14° lower. 



t Til is agrees more nearly with Despretz (p. 245) than with Dr. Marcet. 

 The latter states that sea water decreases in weight to the freezing point until 

 actually congealed. In four experiments Dr. Marcet cooled sea water down to 



