The earth is therefore almost perfectly a smooth and solid ball. It is, however, almost certain that it was not always solid. It is, on the contrary, almost certain that at a period far exceeding in remoteness any time of which mere human indications can be found, the globe of the earth was one mass of liquid matter, heated to a degree exceeding our most intense fires, and wherein were melted all together the various elements which have since arranged themselves into their present forms. From having been thus liquid, the earth, which, revolving on its axis, produces by the side it turns to the sun the alternating day and night, has bulged out where the rotation of the surface is most rapid, at the equator, and has become flattened at the extremities of its axis, at the poles, just as a thin hoop which we spin round becomes compressed. The amount of this flattening is however very small. The equatorial diameter of the earth being accurately 7925, and the polar diameter being 7898, the compression is 27 miles. To account for the existence of this compression, the earth must have been originally liquid, for otherwise the rotation on its axis could not have generated this regular form. If it had been solid when it began to revolve, it should either have retained its original form, or it should have broken in pieces; but certainly unless it had been liquid, it could not have arrived at the exact degree of flattening which its velocity of rotation should have produced in a liquid mass.