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54 sea from London to Ipswich, and have observed Walton Tower rising out of the edge of the waters. Many persons, too, who have gone across the Irish Channel, have seen the mountains on one side disappear, as if they dipped into the sea, and they have also seen the mountains arise out of the sea on the other side, perfect in shape, coming out by degrees, just as if seen rising over the brow of a hill. The inference is, that the water is curved, to produce these phenomena. These are to be seen in the course of ordinary expeditions; but those who voyage further, those who have gone to the Cape of Good Hope, know that, as they go on, every night they lose sight of our stars by degrees, and other stars come up on the other side. In a southern latitude they lose the northern stars, and they get more of the southern stars. All this leads us to the conclusion that the earth is something curved. Again, people have sailed round the earth. This was done for the first time by Magellan and his successors in command: and for the second time by Sir Francis Drake. From the time of Sir Francis Drake, this has been done every year; ships are indeed almost daily prosecuting such voyages. It is a common thing for ships to sail in an easterly direction to Australia, and to return by continuing their eastward course, and not by coming back the same way they set out. The earth, therefore, roughly speaking, is something round, and there are limits to its extent.

Now, the question is, what is its extent? Having got a measure of considerable length by such a process as I have described, how can we use that to determine what is the size of the earth? In order to explain this, suppose Figure 18 to represent a slice of the earth, curved as a slice of the earth would be.