obedience to his will, and are subject to the laws *he imposes on them. Thus celestial and sublunary nature are the same; order and regularity result from seeming confusion, and subordination and dependence are to be seen in every part of the universe.

This illustrious philosopher had made his discoveries in geometry, and laid the foundation of his two celebrated performances, the Principia and the Treatise on Optics, when he was only twentyfour years of age; which is a circumstance no less extraordinary than the discoveries themselves; and serves to countenance the idea of Fontenelle, who observes, upon this occasion, that if intelligent beings, of an order superior to man, make a progress in knowledge by certain gradations, they probably fly whilst we creep, and pass over, without notice, many of the intermediate steps, which the confined limits of the human mind render absolutely necessary to our advancement.

"When we consider, says this ingenious writer, that, according to the doctrine of Newton, every single satellite of Saturn must gravitate towards the other six; the other six towards the seventh; all the seven towards Saturn; and Saturn and all of them towards the sun, according to a particular law what an immense skill in geometry must have been requisite to unravel the intricacies of so many different relations. It was a daring attempt to undertake it; and one cannot perceive, without amazement, that from so abstracted a theory, formed of so many particular theories, and

each of them perplexed with innumerable difficulties, conclusions should always arise exactly conformable to fact and experience." These are, certainly, such instances of genius and penetration, that, when taken in their fullest extent, the idea of the poet will scarcely be thought too extravagant :

"Nature and Nature's laws lay hid in night,
God said, Let Newton be, and all was light.".

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LETTER VIII.

OF THE NATURE OF THE TIDES.

In my last letter I have unfolded to you the grand principle of attraction, and the manner in which it. operates. We have seen the genius of Newton in the heavens, and travelled with him to the sun and the planets. Let us now descend, and follow him into the world of waters, through the depths of the ocean. By what power or cause is it, that this vast liquid body rises and falls alternately, twice a day, in a manner so constant and regular. The ancients considered it as one of the greatest mysteries in nature, and were utterly at a loss to account for it.

Aristotle, the great oracle of antiquity, is represented as having thrown himself into the sea, because he was unable to explain its motions; and when he was in India, with Alexander the Great, it is said that he wanted to follow the tide in its reflux, to see where it would go. The story is sufficiently absurd; but not more so than the following one related of Kepler. He, in one of his reveries, considered the earth as a living being, and thought the flux and reflux of the sea was the effect of its respiration: men, and other creatures, he conceived to be insects which feed upon this animal; bushes and trees the bristles on his back; and the water of seas and rivers a liquid which circulates in his veins.

Galileo, Des Cartes, and even Kepler, have,

however, expressed themselves more philosophically upon this subject; but the first who clearly pointed out the cause of the phænomenon, and showed its agreement with the effects, was Newton. To a genius like his, enterprise and discovery were recreation. The moon he presently saw was the principal agent which produces these motions; and, by applying his new principles of geometry and attraction to the enquiry, he soon showed the manner in which they are effected. To follow him through all his calculations, would be to perplex the subject instead of elucidating it. Not to insist, therefore, upon abstruse investigations, which are intelligible only to mathematicians and philosophers, I shall begin by describing the most obvious facts, and afterwards show their conformity with the theory he has established.

The ocean, it is well known, covers more than one half of the globe; and this large body of water is found to be in continual motion, ebbing and flowing alternately, without the least intermission. What connection these motions have with the moon, we shall see as we proceed; but, at present, it will be sufficient to observe that they always follow a certain general rule. For instance, if the tide be now at high-water-mark, in any port, or harbour, which lies open to the ocean, it will presently subside, and flow regularly back, for about six hours, when it will be found at low-water-mark. After this, it will again gradually advance for six bours, and then return back, in the same time, to its former situation; rising and falling alternately,

twice a day, or in the space of about twenty-four hours.

The interval between its flux and reflux, is, however, not precisely six hours, but about eleven minutes more; so that the time of high water does not always happen at the same hour, but is about three quarters of an hour later every day, for thirty days, when it again recurs as before. For example, if it be high water to-day at noon, it will be low water at eleven minutes after six in the evening; and, consequently, after two changes more, the time of high water the next day, will be at about three quarters of an hour after noon; the day following it will be at about half an hour after one; the day after that at a quarter past two; and so on for thirty days; when it will again be found to be high water at noon, the same as on the day the observation was first made: which exactly answers to the motion of the moon; she rises every day about three quarters of an hour later than upon the preceding one; and, by moving in this manner round the earth, completes her revolution in about thirty days, and then begins to rise again at the same time as before.

To make the matter still plainer; suppose, at a certain place, it is high water at three o'clock in the afternoon, upon the day of the new moon; the following day it will be high water at three quarters of an hour after three; the day after that at half an hour past four; and so on, till the next new moon; when it will again be high water exactly at three o'clock, the same as before. And by observ