from bottom to top, the seas of each hemisphere, in thermal alternation with the seasons, were raised to summer 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 pole, the ebb and flow of a great thermal tide that would turn with the sun in the ecliptic, and tell the equinoxes by 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 difference 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 water from one hemisphere 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 thermal 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, 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 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 temperate zones; it then dips again as it recedes towards the region of perpetual winter.
440. The isothermal floor of the ocean.—The observations of Kotzebue, Admiral Beechey, and Sir James C. Ross first suggested 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 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
