Boyal Physical Society. 



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have disclosed interesting facts concerning its depth and temperature ; 

 these were followed by other British and American navigators, and the 

 whole has been collected and published by the labours of Lieutenant 

 Maury. From these and other researches we find that the earth's super- 

 ficies both under the ocean and above it presents an exceedingly irregular 

 form, consisting of a series of elevations and depressions. On this irre- 

 gular surface are diffused the waters of the ocean, spreading over and 

 concealing from view more than two-thirds of the superfices, while the 

 higher portions only, amounting to somewhat less than one-third, appear 

 as dry land. The greater amount of soundings have been made in the 

 Atlantic Ocean, and thus we have become better acquainted with its 

 bottom than with that of the other oceans of the globe. A section from 

 the Cape de Verd Islands, on the coast of Africa, to the mainland of South 

 America, gives soundings of 17,000 and 22,800 feet. A section south of 

 Newfoundland gives the greatest depth yet authentically ascertained as 

 27,180 feet. Further north in the line of the late electric telegraph from 

 Ireland to Newfoundland, the depths are 10,000, 11,000 and 12,000 feet. 

 On taking a mean of twenty-seven soundings in various parts of the At- 

 lantic, and rejecting a few doubtful ones, the mean depth of this ocean is 

 indicated as 13,100 feet, or two and a half miles. Soundings in the 

 Pacific, also, indicate depths equal to the above in some positions, but the 

 probability is that the central portions of that vast ocean, occupied by exten- 

 sive coral reefs and innumerable islands, are of less depth than the mean 

 of the Atlantic. The Mediterranean exhibits depths of 5000, 10,000, and 

 south west of Malta, 15,000 feet. The North Sea is shallow. On the 

 whole an approximate mean of the ocean depth may be estimated at two 

 miles. Now, if we turn to the elevations of dry land we find that a few 

 mountain peaks attain heights equal, if not surpassing, the extreme de- 

 pressions of the ocean, in the Andes, 20,000 to 23,000 ; in the Himalaya, 

 Kin-Kinchunga, 28,000 feet; and Mount Everest, 29,000 feet. But the 

 mean elevation of land is far inferior to the mean depth of ocean. 

 According to Humboldt's calculations, were the whole surfaces of the con- 

 tinents of Asia, America, and Europe reduced to a uniform level, that 

 mean level would stand at only 1000 feet above the sea-level. From 

 recent explorations of Africa, by Beke, Livingstone, and Speke, we find 

 that extensive table lands of 2000 to 3000 feet occupy the central por- 

 tions. Some mountain peaks attain a height of 20,000 feet, and these, 

 contrasted with the vast level deserts and low lying river valleys and 

 shores, would seem to indicate not a higher general level for Africa than 

 that of the other continents. Similar recent explorations in Australia 

 also indicate that that region may be also comprehended in a general 

 mean elevation of 1000 feet of the whole dry land of the globe. We thus 

 find, that while the extremes of elevation of land about equal the extreme 

 depressions of ocean, the mean depth of the ocean is 10,560 feet, or 

 two miles, while the mean elevation of land is only 1000 feet. If we add 

 these two means together, we have 11,560 feet as the mean of the irregu- 

 larities of the earth's surface. Now, it will be perceived that the ocean 

 surface does not stand at the mean or half section of these irregularities, 

 but on the contrary, stands at the base of the dry land, or 1000 feet from 

 its mean upper surface, and nearly 5000 feet above the line which we 

 have indicated as the mean of the earth's irregularities ; for if the ocean 

 were entirely awanting, the line would, in reality, be the mean line of 

 the earth's irregularities, and consequently the true circumference and 

 central line of gravity regulating the invariable diurnal revolution of the 

 spheroid. Or to extend the expression of the formula used by Humboldt, 

 if the whole irregularities of the earth's surface were levelled down to 



NEW SERIES. VOL. XTX. NO. II. APRIL 1864. 2 T 



