150 Proceedings of the Royal Physical Society. 



depths equal to the above in some positions, but the pro- 

 bability is that the central portions of that vast ocean, occu- 

 pied by extensive coral reefs and innumerable islands, are 

 of less depth than the mean of the Atlantic. The Mediter- 

 ranean 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 depressions of the 

 ocean, — in the Andes, 20,000 to 23,000 feet; 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 calcula- 

 tions, were the whole surfaces of the continents 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 portions. 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 ex- 

 treme 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 irregularities 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 bb (see woodcut), 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 



