218 THE PHYSICAL GEOGRAPHY OF THE SEA. 



hemisphere to the other. This double disturbance of equilibrium 

 would not give rise to a tidal wave — mere motion without trans- 

 lation — but to a tidal flow and reflow of water from one hemi- 

 sphere to the other in volumes of vast magnitude, power, and 

 majesty. This is an exaggerated view of the dynamical force of 

 the sunbeam ; but it is presented to show the origin of the ther- 

 mal tide shown on Plate IV. The difference between the actual 

 and the supposed thermal tides is one of degree merely ; for the 

 sea water that is liable to any considerable change of temperature, 

 instead of reaching from the bottom to the top, is scarcely more 

 than a " pellicle" to the ocean. Nevertheless, it is a regular peri- 

 odical flow and reflow between the poles and the equator. It is 

 the annual ebb of this tide which fills the upper half of the North 

 Atlantic with icebergs every spring and summer. The heated 

 portion forms a stratum or layer which is thickest at the equator, 

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

 perate zones ; it then dips again as it recedes toward the region 

 of perpetual winter. 



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

 The isothermal floor J^mcs C. Koss first suggcstcd the existcncc in the 

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



cording to Kotzebue, is SQ°. The depth of this bed of water of in- 

 variable and uniform temperature is computed to be 1200 fathoms 

 at the equator. It gradually rises thence to the parallel of about 

 56° IST. and S., where it crops out, and there the temperature of the 

 sea, from top to bottom, is conjectured to be permanently at S6°, 

 The place of this outcrop, no doubt, shifts with the seasons, vibrat- 

 ing 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 w^ater 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 whicb the surface temperature is below, and one medial 

 zone in whicb it is above 39°.5,* being 80° at the equator ; and 



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



