That such attempts would be made was inevitable.  As soon as men began to reason on the subject at all, they could not fail to discredit the assertion that the earth is an indefinite plane.  No one can doubt that the sun we see to-day is the self-same sun that we saw yesterday.  His reappearance each morning irresistibly suggests that he has passed on the underside of the earth.  But this is incompatible with the reign of night in those regions.  It presents more or less distinctly the idea of the globular form of the earth.

The earth cannot extend indefinitely downward; for the sun cannot go through it, nor through any crevice or passage in it, Since he rises and sets in different positions at different seasons of the year.  The stars also move under it in countless courses.  There must, therefore, be a clear way beneath.

To reconcile revelation with these innovating facts, schemes, such as that of Cosmas Indicopleustes in his Christian Topography, were doubtless often adopted.  To this in particular we have had occasion on a former page to refer.  It asserted that in the northern parts of the flat earth there is an immense mountain, behind which the sun passes, and thus produces night.

At a very remote historical period the mechanism of eclipses had been discovered.  Those of the moon demonstrated that the shadow of the earth is always circular.  The form of the earth must therefore be globular.  A body which in all positions casts a circular shadow must itself be spherical.  Other considerations, with which every one is now familiar, could not fail to establish that such is her figure.

But the determination of the shape of the earth by no means deposed her from her position of superiority.  Apparently vastly larger than all other things, it was fitting that she should be considered not merely as the centre of the world, but, in truth, as—­the world.  All other objects in their aggregate seemed utterly unimportant in comparison with her.

Though the consequences flowing from an admission of the globular figure of the earth affected very profoundly existing theological ideas, they were of much less moment than those depending on a determination of her size.  It needed but an elementary knowledge of geometry to perceive that correct ideas on this point could be readily obtained by measuring a degree on her surface.  Probably there were early attempts to accomplish this object, the results of which have been lost.  But Eratosthenes executed one between Syene and Alexandria, in Egypt, Syene being supposed to be exactly under the tropic of Cancer.  The two places are, however, not on the same meridian, and the distance between them was estimated, not measured.  Two centuries later, Posidonius made another attempt between Alexandria and Rhodes; the bright star Canopus just grazed the horizon at the latter place, at Alexandria it rose 7 1/2 degrees.  In this instance, also, since the direction lay across the sea, the distance was estimated, not measured.  Finally, as we have already related, the Khalif Al-Mamun made two sets of measures, one on the shore of the Red Sea, the other near Cufa, in Mesopotamia.  The general result of these various observations gave for the earth’s diameter between seven and eight thousand miles.