The main body of the sea has a much greeter depth. In most profs of the Atlantic, where the sea has been sounded, no bottom was found with 300 fathoms. Between Europe and America the bottom seems to offer great inequalities, being furrowed by deeper tracts, which run north and south. In the North Atlantic are the greatest depths at which the bottom of the sea has been reached; the places where it has been fathomed, according to Maury, are not deeper than 23,000 feet, or about four miles and three quarters. The deepest place in this ocean is probably between the parallels of and N., and immediately to the southward of the Grand Banks of Newfoundland. The small general depth of this part of the Atlantic has given rise to the opinion that the sea round the North Pole is not so deep as that which sur rounds the South Pole, but this opinion has no foundation. Phipps and Scoresby sounded in several places between Spitzhergen and Greenland with from 780 to 1200 fathoms, without finding a bottom ; and Ellis and Ross did the same in Hudson's Bay and Baffin's Bay. In the Pacific, Ocean the depth seems also to be very considerable, but few soundings have been made there.
Tho depth of the sea near the land varies with the nature of the shores. Where the country near the sea is elevated, and terminates in high and rocky shores, the sea is generally of considerable depth, as in the fiords of Norway. Such shores have generally good and safe harbours. But when If low plain terminatts, on the sea with a flat sandy bank, the sea is shallow and frequently continues to be so to a great distance (rein the shore. In many places such shores are inaccessible even to boats, and vessels must keep at a distance of many miles. Such shallows consist either of sand or of mud. These low shores are generally destitute of harbours, or at least they occur only at great distances, and much expense is required to maintain them in an efficient state : the harbours on rocky coasts are not subject to this inconvenience.
It is a very remarkable phenomenon, which has not yet been satis factorily explained, that the temperature of the sea decreases as the land is approached, and it also decreases on shoals and banks ; and as this decrease may be detected by the thermometer at a considerable distance from land, this instrument is now used for the purpose of ascertaining the approach to land or the presence of shoals. It must, however, be observed, that though this decrease generally takes place, it is not universal.
As late as the end of the last century it was a generally received opinion that the whole mass of sea-water, from the surface to the bottom, had the same temperature in the same latitude. But nume rous observations, which have been more recently made, have shown the inaccuracy of this assumption. It has been found that the law which is constant for the earth must be inverted for the sea. The farther we descend into the interior of the earth, the higher is the tem perature; but the deeper we dip into the sea, the lower is the tempe rature of the water. But this does not take place in the same ratio iu the whole sea. Captain Ross found the temperature of the sea in Baffin's
Bay, 3900 feet below the surface, + 25'52 , while the surface itself was + Beechey, in 47' 18' N. lat., at &depth of 5124 feet, found the water + and at the surface it was 46'22°. Sabine found that the water in the Caribbean Sea, at a depth of 1000 fathoms, had a temperature of 45'50°, whilst at the surface it was 83'30°. According to an observation of Franklin, the water at a depth of 650 fathoms, in 44', was 40'5'; at 450 it was 41'; and at the surface, 45'. There are a few instances in which a new increase of the temperature has been observed at a very great depth. In the following observations made by Beeches', in the Pacific, the temperature became stationary at a great depth As a proof of the increase of the temperature of the sea at a great depth, we copy the following observations, of which the first two were made by Beechey, and the last by Prescott; the first in the Pacific, and the last two in the Atlantic Ocean :— The renewed increase at a great depth is a very difficult problem to solve : it is possible that it may be owing to submarine currents of different degrees of temperature, as some have supposed. Sir James C. Ross ascertained, by seven different experiments, that between the parallels of 55 and 68° 30' S., there is a belt encircling the earth, where the mean temperature of tho sea, that of the greatest density of its water, or + 89.5°, prevails throughout its entire depth, forming a neutral border between two great thermic:, basins of the ocean.
But there are facts on record which clearly show that in certain parts of the ocean there must exist some agency by which the water acquires a higher degree of temperature than might have been expected from ordinary causes. Horner, io Krusemstenee 'Travels; observes that, in some places in the Gulf-stream, the hand-lead, when it had descended to a depth of between 480 and 600 feet, was heated to such a degree that it was impossible to take it into one's hand. A still more remarkable anomaly is presented by the temperature of the sea between Greenland and Spitzhergen. In nearly every trial, Scoresby found that this sea, at a depth of from 100 to 200 fathoms, was from 6° to warmer than at the surface ; and Franklin Oates that when he accom panied Captain Buchan in his expedition to the North Pole, the water brought from any great depth was invariably found to be warmer than that of the surface. Some persons are of opinion that the melting of the great masses of ice, by which that sea is surrounded and partly covered even in summer, may have had the effect of cooling the surface. But this is contrary to the well-established law that the colder water, being the denser, sinks to the bottom, and the warmer rises to the surface ; and further, it may be asked why Ross and Parry, ln navi gating Davis's Strait, Baffin's Bay, and Hudeon's Bay, where the masses of ice are neither lees numerous nor loss extensive, always found the contrary to take place.