212 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



and which comes to tlie surface near the polar edge of the tem- 

 perate zones ; it then dips again as it recedes towards the region of 

 perpetual winter. 



440. The ohservations of Kotzebue, Admiral Beechey, and Sir 

 The isothermal floor Jamcs C. Eoss first suggcstcd the oxistence in the 

 of the ocean. occan of this isothcrmal floor. Its temperature, ac- 

 cording to Kotzebue, is 36°. The depth of this bed of water of 

 invariable and uniform temperatm-e is 1200 fathoms at the equa- 

 tor. It gradually rises thence to the parallel of about 56° N. and 

 S., when it crops out, and there the temperature of the sea, from 

 top to bottom, is conjectured to be permanently at 36°. The 

 place of this outcrop, no doubt, shifts with the seasons, vibrating 

 up and down, i.e.-^ north and south, after the manner of the calm 

 belts. Proceeding, in our description, onward to the frigid zones, 

 this aqueous stratum of an unchanging 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 thi-ee great regions — two polar 

 basins in which the surface temperatm-e 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 in- 

 suring an increase of density in descending within much wider 

 limits of temperature than here contemplated." 



441. The temperatm-e of 39°. 5 was assigned to this floor pro- 

 Thprmai dilatation ' bably uudcr tlic suppositiou that sea water follows 

 of the ^-ater. fresli iu 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 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 : 



* This remark was made by Sir John on the supposition, probably, that the maxi- 

 mum density of sea water was at the same temperature as that of fresh, but it is 

 the same 12 ' or 14^ lower. 



