place before it completes its revolution, which is a siderial year. For example, (Pl. IX, fig. 1,) if the sun S, earth E, and a star be in the same straight line at an equinox, the earth revolving through a, will not be at E at the same equinox, but somewhere at e. Hence it must revolve farther, from e to E, before it completes its revolution; and the time of doing this is the difference between a solar and siderial year, and amounts to about 20 minutes. The distance e E is about 50′′ of a degree annually, and constitutes what is called the precession of the equinoxes.

191. The precession of the equinoxes is to be accounted for in much the same way that the retrograde motion of the moon's nodes is. It has been stated, that the diameter of the earth at the equator is greater than through the poles. Suppose this excess of matter about the equator to be a ring round the earth, but separate from it, leaving the earth a perfect globe or sphere. Let A b Bc (Pl. IX, fig. 2,) be a circle in the plane of the ecliptic. Let ACB be half the ring we have supposed, lying above the ecliptic, and making an angle with it of 2340. Now the effect of the sun's attraction on this ring is the same, during a year, whether we suppose the earth to move round the sun, or the sun to move round the earth. Let us then suppose the sun to move round the earth in the circle a SV. While the sun is moving from a through S to V, that is, during half the year, the sun acts successively on all the parts of the ring from A though C to B. This action tends to draw the ring into the plane of the ecliptic; and the effect is such as to make it cut the ecliptic somewhere at x, and not at B, where it did before. So while the sun is going the other half of its orbit, it acts in the same manner on the other half of the ring; and makes it cut the ecliptic somewhere at d instead of A. Thus the equinoxes are constantly

shifting backward. For the effect we have supposed on this ring, detached from the earth, actually takes place while it is attached to the earth, and forms a part of it.

192. As the equinoctial points move backward, and the sign Aries always begins at one of them, and all the other signs of 30° each follow Aries in order, it follows that all the signs of the ecliptic or zodiac move backward with the equinoxes. Consequently stars, which are in one sign at one time, will be in the succeeding one at another. Hence comes the fact spoken of No. 54. The sign Aries nearly coincides with the constellation Pisces; and Taurus with the constellation Aries, and so on. When these names were given to the signs: and constellations, probably each sign coincided with the constellation of the same name; but on account of the precession of the equinoxes, there is now about one sign or 30° difference. In about 2000 years there will be a difference of two signs or 60°.

SECT. VI.

Of the Tides,

193. Oceans are observed to have a regular risingand falling of their waters, which are called tides. There are two tides in about 25 hours. These are occasioned by the attraction of the moon; but affected also by that of the sun.

194. Let M (Pl. IX, fig. 4,) be the moon revolving in its orbit; E the earth covered with water. The moon, drawing the earth to itself, affects the solid parts. of it, just as if its whole weight were in a single point in or near the centre E. Now the waters at A are nearer the moon than the point E, and are consequently

more attracted than the earth. Hence the waters are heaped up under the moon at A. But the waters on the opposite side at B are less attracted than the earth; consequently the earth is drawn away from them, and they are heaped up at B. When the waters are heaped up at A and B, it is plain they must recede from the intermediate points C and D.

195. Thus while the earth turns on its axis, any particular place as A has two tides, while passing from under the moon till it comes under the moon again. But while the earth is turning on its axis, the moon advances in its orbit, so that the earth must a little more than complete its rotation before the place A comes under the moon. This makes high or low water at any place about 50 minutes later one day than on the preceding.

196. It is obvious, that the waters directly under the moon are nearer to it than those any where else ; consequently are more attracted. And as the moon's orbit differs but little from the ecliptic, the moon cannot be but about 29° from the equator, generally it is much less. Hence the waters about the equator are more attracted, and of course the tides are higher than towards the poles. At or near the poles tides must

cease.

197. The sun attracts the waters as well as the moon. But the difference between the distance of the centre and surface of the earth from the sun, compared with the whole distance of the earth from the sun, is so small, that the sun acts on the waters very nearly as it does on the solid land; and consequently produces little tide. When the moon is at full or change, it acts with the sun; that is, the sun and moon tend to raise tides at the same places. Hence tides are then very high, and are called spring tides. But when the moon is in quadrature (Pl. IX, fig. 5,) the sun and moon

tend to raise tides at different places, and counteract each other's effects. The moon raises tides at C and D, and the sun tends to raise them at A and B. But the sun does not raise tides; its only effect is to diminish or increase those of the moon. Tides, when the moon is in quadratures, are very low, and are called neap tides.

198. As the sun is always in the ecliptic, and of course is never more than 2310 from the equator, his influence is joined with that of the moon in making tides high at the equator, and lower towards the poles. Hence, if the earth were a perfect globe, and had no excess of matter nearer the equator, the constant action of the sun and moon on the waters of the ocean would keep the equatorial region constantly immersed.

199. But spring tides are not always equally high at the same place. When the sun and moon are in the equator, their combined effect on the water is greatest. This is at the time of the equinoxes. But as the earth is nearer the sun in winter than in summer, and thereby the sun's action is increased, therefore our highest spring tides are usually a little after the autumnal equinox, and little before the vernal.

200. It is to be noticed, that tides are not at their height when the moon is in the meridian, as would appear from the figures; but this takes place one or two hours after the moon has passed the meridian, because she continues to attract the water during that time.

201. Besides the continually varying, co-operating, or contrary attraction of the sun and moon, there are other causes which affect the time and height of full tide. Strong winds, blowing in a particular direction, and for a long time, produce currents in the ocean, which greatly affect the regular tides. Different places, also equally subject to the moon's action, will have ma

terially different tides; owing to currents in the ocean, to the position of the neighbouring coast, &c. Continents stop the tides in their course from east to west; consequently, tides are generally higher (on an eastern coast than on a western. Thus it is supposed, that the water in the Gulf of Mexico is several feet higher than on the other side of the isthmus; and Napoleon says, (Voice from St. Helena,) "I had the Red Sea surveyed, and found that the waters of it were thirty feet higher than the Mediterranean when the waters were highest, but only twenty-four feet at the lowest." In mouths of rivers and bays opening eastward, and growing narrower inland, tides rise to a great height. At the mouth of the Indus, tides rise thirty feet; and in the bay of Fundy, sometimes to the astonishing height of sixty feet. They are remarkably high on the coast of Malay, in the strait of Sunda, and in the Red Sea. In the Mediterranean and Baltic, which have very narrow inlets, and open westward, scarce any tide is perceptible. Hence the Greeks and early Romans were ignorant that any such phenomenon existed.

202. In narrow rivers, the tides are frequently very high and sudden, from the resistance of the banks. The tide is said to enter the river Severn in England sometimes with a head ten feet in height. In rivers where there are many obstructions arising from banks, shallows, and sinuosities, there are not unfrequently several tides at different places. Thus in the river Thames, it is high tide at London and at the Nore (mouth of the river) at the same time; while between these places, there is low tide. The same, according to Dr. Franklin, takes place in the Delaware river. In the river Amazon, in South America, where the tide flows up 500 miles, it is said there are no fewer than seven high tides at various distances, and of course, low tides between them, all at the same time.