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 hh (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 
