CONVERSATION XVI.

Of the Tides.

TUTOR. We will proceed to the consideration of the Tides, or the flowing and ebbing of the ocean.

James. Is this subject connected with astronomy?

Tutor. It is, inasmuch as the tides are occasioned by the attraction of the sun and moon upon the waters, but more particularly by that of the latter. You will readily conceive that the tides are dependent upon some known and determinate laws, because, if you turn to White's Ephemeris, you will see that the exact time of high water at London'

Tutor. In the position of the earth and moon as it is in our figure, the waters are raised at ▲ by the direct attraction of the moon, and a tide is accordingly produced; but when, by the earth's rotation, T comes, 12 hours afterwards, into the position a, another tide is occasioned by the receding of the waters there from the centre.

James. You have told us, that the tides are produced in those parts of the earth to which the moon is vertical, but this effect is not confined to those parts?

Tutor. It is not, but there the attraction of the moon has the greatest effect in all other parts her force is weaker, because it acts in a more oblique direction,

Charles. Are there two tides in every 24 hours?

Tutor. If the moon were stationary this would be the case; but because that body is also proceeding every day about 13 degrees from west to east in her orbit, the earth must make more than one revolution on its axis before the same meridian is in conjunction with the moon, and hence two tides take place in about 24 hours and 50 minutes.

James. But I remember when we were at the sea, that the tides rose higher at some seasons than at others: how do you account for this?

Tutor. The moon goes round the earth in an elliptic orbit, and therefore she approaches nearer to the earth in some parts of her orbit than in others. When she is nearest, the attraction is the strongest, and consequently it raises the tides most; and when she is farthest from the earth,

her attraction is the least, and the

tides the lowest.

James. Do they rise to different heights in different places?

Tutor. They do: in the Black Sea and the Mediterranean the tides are scarcely perceptible. At the mouth of the Indus the water rises and falls full 30 feet. The tides are remarkably high on the coast of Malay, in the Straits of Sunda, in the Red Sea, along the coasts of China, Japan, &c. In general, the tides rise highest and strongest in those places that are narrowest.

Charles. You said the sun's attrac tion occasioned tides as well as that of the moon.

Tutor. It does; but, owing to the immense distance of the sun from the earth, it produces but a small effect in comparison of the moon's

attraction. Sir Isaac Newton com puted, that the force of the moon raised the water in the great ocean 10 feet, whereas that of the sun raised it only 2 feet. When both the attraction of the sun and moon act in the same direction, that is, at new and full moon, the combined forces of both raise the tide 12 feet. But when the moon is in her quarters, the attraction of one of these bodies raises the water, while that of the other depresses it; and therefore the smaller force of the sun must be subtracted from that of the moon; consequently the tides in the midst of the ocean will be no more than 8 feet. The highest tides are called spring tides, and the lowest are denominated neap tides.

James. I understand, that, in the former case, the height to which the