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28 from the most probable supposition we can make as to the past history of the planet. To show this we may take the case of the Earth. Investigations along several different lines all result in showing that the polar flattening of the Earth is almost exactly such as would result in a fluid body whose density from surface to centre increased according to the pressure and temperature of our Earth in the past, and which rotated with its present angular velocity. In the case of Mars, Tisserand has shown that the polar flattening under the influence of his present rotation would, if the increase of density in his strata were similar to the Earth’s, be 1/227 of his equatorial diameter. If, on the other hand, his mass were homogeneous, his polar flattening would be 1/178. Now, in a fluid condition a body could not remain homogeneous, owing to the pressure exerted by the outer strata upon the inner ones, unless the matter of which it was composed were rigorously incompressible, which is certainly not the case with the Earth, and with quite equal certainty not the case with Mars. On the other hand, the increase of density from surface to centre is undoubtedly less in Mars than in the Earth, since the pressure depends upon the mass and the Earth’s mass is nearly ten times that of Mars. Consequently, from this cause, the polar flattening should be somewhere between 1/178 and 1/227,