Present Problems of Geophysics. — Becker. 5 
solar system compelled the immortal Kant and the ever- 
living Laplace to seek the origin of the planets, the sun and 
the other stars in heterogeneous nebulas which they sup- 
posed to have condensed about one or several nuclei. Every 
attempt to devise an essentially different hypothesis has 
failed, and every history of the globe which begins after the 
birth of the planet is unsatisfying. In the drama of the uni- 
verse there must have been pre-nebular scenes, but of these 
we have as yet no inkling. The nebular hypothesis, as its 
authors propounded it, explains the similarity in the composi- 
tion of the members of the solar system which is indicated 
by the analysis of meteorites and by the spectroscope, though 
the facts thus revealed were unknown to Kant and Laplace. 
It is also compatible with and accounts for the heterogeneity 
in the composition of the earth manifested in the actual 
asymmetric distribution of oceans, mountain ranges and 
anomahes of gravitational force, as well as in the curiously 
local occurrence of certain ores (such as those of tin and 
mercury) and in the predominance of certain alkalies among 
the rocks over wide areas. 
This heterogeneity, however, is of a small order of mag- 
nitude. The general dependence of gravity on latitude, the 
nearly spheroidal shape of the earth and other phenomena 
show that the distribution of density is nearly symmetrical, 
while the divergence of the spheroid from the figure char- 
acteristic of a fluid of the same mean density and mass as 
the earth demonstrates that the interior layers of equal 
density are oblate. These and similar facts are consistent 
with and are strong evidence for the hypothesis that the 
globe has been fused at least to a considerable depth from 
the glowing surface of the gathering nebulous mass. Never- 
theless, Houghton, and more recently professor Chamberlin, 
have supposed that the accretion of nebulous matter was so 
slow that the heat of impact did not suffice to produce 
fusion. The hypothesis of superficial fusion is not incompati- 
ble with the minor heterogeneity pointed out above ; for 
the laws of diffusion in viscous fluids give proof that sensibly 
perfect homogeneity could not be produced even in 50,- 
000,000 years throughout a body of liquid originally hetero- 
geneous and possessing a tenth of the mass of the earth. On 
