Chap. 7 . PHILOSOPHICAL DISCOVERIES. 363

cause the places, where the moon raises most, and most de-presses, the water, are at that distance from each other.Hence it appears, that it is only in the great oceans that suchtides can be produced ; and why in the larger pacific ocean theyexceed those in the Atla?itic ocean. Hence also, it is obviouswhy the tides are not so great in the torrid zone, betweenAfrica and America , where the ocean is narrower, as in thetemperate zones on either side ; and, from this also, we mayunderstand why the tides are so small in istands that are veryfar distant from the shores. It is manifest, that, in the Atlanticocean, the water cannot rise on one stiore but by descendingon the other ; so that, at the intermediate distant istands, itmust continue at about a mean height betwixt its elevation onthe one and on the other lhore.

As the tides pass over shoals, and run through straits intobays of the sea, their motion becomes more various, and theirheight depends on a great many circumstances. The tide thatis produced on the western coasts of Europe , in the Atlantic^ cor-responds to the situation of the moon we described above. Thusit is high w ater on the coasts of Spain , Portugal , and the westof Ireland , about the third hour after the moon has pasted themeridian. From thence it stows into the adjacent channels, asit finds the easiest pasiage. One current from it, for example,runs up by the south os England , another comes in by thenorth of Scotland : they take a considerable time to move allthis way, and it is high water sooner in the places to whichthey first come ; and it begins to fall at those places, whilethey are yet going on to others that are farther in their course.As they return, they are not able to raise the tide, because thewater runs faster off than it returns, till, by a new tide propa-gated from the open ocean, the return of the current is stop'd,and'the water begins to rise again. The tide takes twelve

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