220 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



from bottom to top, the seas of each liemispliere, in thermal alter- 

 nation with the seasons, were raised to siuiimer heat and lowered 

 to winter temperature : the change of sea level from summer to 

 winter, and from winter to summer, in one hemisphere, would, 

 from this cause alone, be upwards of 125 feet; and in its rise and 

 fall we should have, from pole to j^ole, the ebb and flow of a 

 great thermal tide that would turn with the sun in the ecliptic, 

 and tell the equinoxes T)}'' the march on the tide staff of its rising 

 and falling waters. But difference of level would not be all that 

 would give strength and volume to this tide ; difference of specific 

 gravity would lend its weight as so much dynamical force, which 

 ditference would create an upper and under annual tide from one 

 hemisphere to the other. This double disturbance of equilibrium 

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

 translation — but to a tidal flow and reflow of w^ater from one 

 hemisphere to the other in volumes of vast magnitude, power, 

 and majesty. This i^ an exaggerated view of the dynamical 

 force of the sunbeam ; but it is presented to show the origin of the 

 thermal tide shown on Plate IV. The difference betv/cen 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, 

 there is a regular periodical flow and reflow between the poles 

 and the equator. It is the annual ebb of this tide which fills the 

 upper half of the Xorth 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 temperate zones ; it then dips again as it recedes 

 towards tlie region of perpetual winter. 



440. Tlie isothermal floor of the ocean. — The observations of 

 Kotzebue, Admiral Beechey, and Sir James C. Eoss first sug- 

 gested the existence in the ocean of this isothermal floor. Its 

 temperature, according to Kotzebue, is 36°. The depth of this 

 bed of water of invariable and uniform temperature is 1200 

 fathoms at the equator. It gradually rises thence to the parallel 

 of about 50 \N. and S., when it crops out, and there the temjDC- 

 lature of the sea, from top to bottom, is conjectured to be perma- 

 nently 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 descri])tion, 

 onward to the frigid zones, this aqueous stratum of an unchanging 



