PRESIDENTIAL ADDRESS. 437 
unknown, and the effect of which is slow, intermittent, butirresistible.’ Forty-two 
years later Sir George Darwin showed that any ellipsoidal inequality in the figure 
must be gradually destroyed by an irreversible action of the same nature as 
internal friction or viscosity. The same may be said of a state in which the 
centre of gravity does not coincide with the centre of figure when the resistance to 
compression is great enough to keep in check the tendency to gravitational in- 
stability. The state would be changed gradually in such a way as to bring the 
centre of gravity nearer to the centre of figure. A symptom of such changes 
might be the occurrence of great subsidences in the neighbourhood of the Crimea, 
where we found the maximum of the first harmonic. Such subsidences are 
supposed by geologists to have taken place in rather recent times. Symptoms 
of the diminution of the inequalities expressed by harmonics of the second degree 
would be found in the gradual disappearance of seas forming part of the great de- 
pression which was described above as a sort of immense Mediterranean (cf. fig. 7) 
in the destruction and inundation of a continent in the northern Atlantic and in 
a gradual increase of depth of the Southern Pacific. The disappearance of seas 
from a vast region surrounding the present Mediterranean basin, and containing 
the Sahara and Southern Asia as far east as the Himalayas, is one of the best 
ascertained facts in geological history ; and the belief in the destruction of a north 
Atlantic continent is confidently entertained. In parts of the Southern Pacific 
a depression represented bySharmonics of the third degree is superposed upon an 
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Fig.15. 
elevation represented by harmonics of the second degree, and we should therefore 
expect to find the depth of the ocean to be increasing gradually in this region. The 
region in question is that of the coral reefs and coral islands, such as Funafuti, and 
the result is in accord with Darwin’s theory of the formation of coral reefs. So far as 
the general distribution of the mass within the earth is concerned, the reduction of 
the inequalities of the first and second degrees would seem to have already proceeded 
very far; for we are assured by geodesists that harmonics of the first degree, and 
those of the second degree which do not represent the effect of the rotation, are far 
from prominent in the figure of the geoid—much less prominent than we found 
them to be in the distribution of continent and ocean. We infer that the inequali- 
ties of the first and second degrees must have been progressively diminished in 
comparison with those of the third degree. The general result of such changes 
would be a gradual diminution of the depths and extents of the oceans which 
correspond with the harmonics of the first and second degrees, and a compensating 
increase in the depths and extents of the oceans which correspond with the harmonic 
of the third degree. To see the character of the changes which would thus be 
brought about, we may examine a figure which shows the composite elevations 
and depressions that are represented by harmonics of the first and second degrees, 
and, separately, those which are represented by harmonics of the third degree. 
In fig. 15 the composite elevations of the first and second degrees are shaded 
vertically, and the elevations of the third degree are shaded horizontally. The 
deep parts of the Atlantic that border the coasts everywhere from Brazil to 
Ashanti are regions in which a depression represented by the third harmonic is 
