36. We have treated these subjects at length in the First Book of the Geography. At present we shall make a few remarks on the operations of nature and of Providence conjointly.—On the operations of nature, that all things converge to a point, namely, the centre of the whole, and assume a spherical shape around it. The earth is the densest body, and nearer the centre than all others: the less dense and next to it is water; but both land and water are spheres, the first solid, the second hollow, containing the earth within it.—On the operations of Providence, that it has exercised a will, is disposed to variety, and is the artificer of innumerable works. In the first rank, as greatly surpassing all the rest, is the generation of animals, of which the most excellent are gods and men, for whose sake the rest were formed. To the gods Providence assigned heaven; and the earth to men, the extreme parts of the world; for the extreme parts of the sphere are the centre and the circumference. But since water encompasses the earth, and man is not an aquatic, but a land-animal, living in the air, and requiring much light, Providence formed many eminences and cavities in the earth, so that these cavities should receive the whole or a great part of the water which covers the land beneath it; and that the eminences should rise and conceal the water beneath them, except so much as was necessary for the use of the human race, the animals and plants about it. But as all things are in constant motion, and undergo great changes, (for it is not possible that things of such a nature, so numerous and vast, could be otherwise regulated in the world,) we must not suppose the earth or the water always to continue in this state, so as to retain perpetually the same bulk, without increase or diminution, or that each preserves the same fixed place, particularly as the reciprocal change of one into the other is most consonant to nature from their proximity; but that much of the land is changed into water, and a great portion of water becomes land, just as we observe great differences in the earth itself. For one kind of earth crumbles easily, another is solid and rocky, and contains iron; and so of others. There is also a variety in the quality of water; for some waters are saline, others sweet and potable, others medicinal, and either salutary or noxious, others cold or hot. Is it therefore surprising that some parts of the earth which are now inhabited should formerly have been occupied by sea, and that what are now seas should formerly have been inhabited land? so also fountains once existing have failed, and others have burst forth; and similarly in the case of rivers and lakes: again, mountains and plains have been converted reciprocally one into the other. On this subject I have spoken before at length,[864] and now let this be said: