TUTOR. And it is as certain that it is so east and west: for, navigators have often sailed round it steering the same course: that is, if they sail an easterly or westerly course at setting off, by continuing the same course they will return to the port whence they departed. This you know they could not do if it were not round, any more than an insect could, by crossing a round table, arrive at the place it set out from; but, by going round the edge it would be still going forward and come again to the point it had left. PUPIL. It is very evident. TUTOR. Again. In every direction, if a ship be seen at a distance, the first things observed are the top-mast and rigging, whilst the hull or body of the ship is hidden behind the convexity, that is roundness of the water, just as you would see a man coming over a hill, you would first see his head, he would be rising more and more to your view till he arrived at the top, where he would be full in sight. PUPIL. I am at a loss to account for the convexity of the water. How can its surface be round? TUTOR. Have you never observed the drops of water falling from the eaves of a house? PUPIL. Often, Sir. TUTOR. Of what shape were they? PUPIL. Globular.—But what is the cause of their being so? TUTOR. Attraction.—For as every particle of water which composes the drop tends to the same center, every part of the surface must be equidistant from the center, it must therefore be spherical. In like manner if you separate quicksilver, each portion will form itself into a globe. PUPIL. All this is very clear. And, for the same reason, the water in the ocean must be convex; for, I remember you told me that it gravitated towards the center of the earth. TUTOR. Once more.—I think you must have seen an eclipse of the moon. PUPIL. I have, Sir. TUTOR. Of what figure was the darkened part? PUPIL. Circular. TUTOR. Take this ball, and hold it before the candle between your finger and thumb, so that the shadow may be thrown on the wall, and in all positions you will find it circular. PUPIL. It is so. TUTOR. Apply this crown piece in the same manner, with the flat side to the candle. PUPIL. It is a circle. TUTOR. Turn it a little obliquely. PUPIL. It is now an ellipsis. TUTOR. Now turn the edge to the candle. PUPIL. The shadow is a strait line. TUTOR. You now see that no other body than that of a globe can in all positions cast a circular shadow. PUPIL. I do, Sir. TUTOR. The darkness on the disc of the moon at the time of an eclipse is the shadow of the earth, which in all situations is circular; the earth, therefore, which casts the shadow, must be a globe. PUPIL. It must be so.—But—— TUTOR. The earth is mountainous.—It is so: but remember that the highest mountain bears no greater proportion to the bulk of the earth than the small irregularities on the peel of an orange bears to that fruit: that objection therefore is soon removed. And yet it is not a true sphere. PUPIL. What then?