The general phenomena of the tides, during one lunar month, or from any one new moon to the next following, will, therefore, be these: At the new moon, the tide will be high, and it will take place at twelve o'clock; next day it will be a little lower, and about 50 minutes later; and it will go on diminishing in height, and happening later and later, till, at the expiration of seven days and three-eighths, it will happen at six o'clock in the evening. From this period to 143 days, it will gra dually increase; and, at the end of that time it will be of the same height, and take place at the same hour as at the time of a new moon. From the full moon to the end of the third quarter, the tide will continue diminishing; at that time it will happen at six o'clock; and during the fourth or remaining quarter, it will continue to increase, till, at the end of the lunation, it be of the same height, and happen at the same hour as at the beginning.

If the earth were covered with an uniform depth of water, if the sun and moon always appeared over head at the equator, if their distances from each other and from the earth were always the same, and if the water had no inertia so that the sun and moon could act upon it and produce their effect instantaneously, that which has been stated would be a permanent description of the tides and a perfect explanation of their causes. But neither of the circumstances which have been now enumerated is strictly true: a considerable portion of the earth's surface is land, formed and distributed without any regularity; and the water by which the remainder of its surface is covered is of very unequal depth; the sun and moon are sometimes both over head at the equator, sometimes, the one of them is on it, and the other to the north or the south, sometimes they are both on the same side of it, and sometimes the one is upon the one side of it and the other upon the other; the relative distances of the sun and moon are constantly varying; and the inertia of the water demands some time before either luminary can produce its effect. All these circumstances, operating, as they do, at the same time, make the real appearances of the tides very different from what the simple theory would lead us to suspect; but these differences, instead of forming any objection to the theory are so many confirmations of it, in as much as they, too, are necessary results of that law of universal gravitation by which the tides, like all the other great phenomena of the system, are explained.

Leaving the disturbance produced by the variable distances of the sun and moon out of the question, because it is not very great in itself, and because the explanation of it would be both long and intricate, it may not be improper to notice the general effect produced by the remaining causes of disturbance. These are, first, the inertia of the waters; secondly, the obliquity of the orbits of the earth and moon; and

thirdly, the interruptions of the land and the inequalities of the bottom of the sea.

1. The inertia of the waters.-The effect of this will be understood, if we consider, that when any portion of matter has been put in motion by any particular force, it continues to move after that force has ceased to act; as, a bullet continues to fly, though the explosive force of the gunpowder ceases to act the moment that the bullet escapes from the piece. Owing to this cause, the water will continue to rise, toward any meridian, for some time after the luminary has passed; and, generally speaking, high-water takes place when the moon is about twenty degrees. westward of the meridian; that is, about one hour and twenty-three. minutes after the moon is south. The same cause affects the spring and neap-tides; the former not happening exactly at the new and full moons, or the latter at the quarters, but taking place a day or two later. These consequences would be produced by the inertia of the waters alone, although the sun and moon were always over the earth's equator, and the whole globe were covered with an uniform depth of water; and they are farther varied by those circumstances.

2. The situation of the luminaries, with regard to the equator, produces variations in the tide. If they be both upon the equator, there will be continual low water at both poles, and the difference between high and low water will increase as one approaches to the equator. If they be both on the one side of the equator, the upper tides in the hemisphere where they are will be higher than in the other; if they be on opposite sides, the total height of the tide will be diminished in proportion to the distance that they are from each other; and, generally speaking, the tides will always be the higher the more nearly the sun, moon, and earth are in the same straight line. The moon's mean declination northward or southward of the sun is never more than about two degrees and a half; so that during the summer half year, the upper spring-tides, that is, those that happen at the new moons, during the day, will be higher in the northern hemisphere than those that happen during the night; while at the full moons the tides that happen during the night will be higher than those that happen during the day; but, during the winter half year, these circumstances will be reversed; and at the same times of the equinoxes, that is, as soon after the 23d of March and of September, as the joint influence of the sun and moon has produced its effect; the tides, generally speaking, will be higher than at any other time of the year.

3. But the greatest cause of disturbance, and the one which makes the time of the tides, at any one place, a matter of observation and not of theory, is the resistance made by the shores of the land; because this sometimes occasions tides of many times the height that theory

would allow, sometimes tides of very unequal duration, sometimes double tides, and sometimes no tides at all.

It will easily be perceived that, in order to produce one complete and perfect tide, a whole hemisphere of the earth would require to be covered with water, and the sun and moon directly over its centre. But nowhere, and at no time, can this happen; because nowhere and at no time does the earth present such a hemisphere to either of the luminaries. The portion of a tide, which can anywhere be produced must, other things being equal, be in proportion to the extent of water. Thus, though the water in a lake, a mill-pond, or even a bowl, is affected by the same force as that which causes a tide in the Atlantic or the Pacific, the effect in the former cases, even where the length of the lake (or inland sea, as it happens) is more than a thousand miles, is not perceptible.

But the shores of the land change the direction of the tides, as well as the height. The natural set of the tide is from east to west, at the same rate as the moon's apparent motion; and if the shore of any large portion of land form an angle with this, the part which is the most easterly must have high water before the rest, and this must turn the high water along the shore. It must do this whether the tide be an actual transfer of water by a current, or merely an undulation of the surface, or partly the one and partly the other. If one wide sea opens into another by a narrow channel, the tide must flow through that channel with considerable violence; and there are instances in which this produces an actual cataract, or fall; the one way when the tide is flowing, and the other when it is ebbing. When the tide flows rapidly against an oblique shore, and finds no outlet, it may make a complete revolution. Thus the tide, which in the regions of the equator flows westward from the shores of Africa to the shores of America, performs a complete circuit in the North Atlantic, passing northward along the shores of America, eastward along the edge of the polar ice, and southward along the shores of Europe. In consequence of this, there are great inequalities in the times of its happening, and also in the height to which it rises. For instance, it is, generally speaking, high water on the western coast of England at the same time that it is low water on the eastern; the tide on the eastern coast sets southward, while that on the south coast sets eastward, and meets the other at the strait of Dover, where there are sometimes double tides in consequence. When two of those currents of tide meet together, and set against a shore, or into a bay or the estuary of a river, they cause a much more elevated and violent tide than can be produced by any other means. There are examples of this on the coast of France, about Brest, in the Bristol Channel, more especially in the river Avon, in the Solway Firth, and in many other places, where the tide rises to a great height, and that with much vigs

lence and rapidity. Indeed, when the tide forms any kind of circuit in an inland sea, it may be perceptible at the bend of a confined bay, though there should happen to be no visible tide in the sea itself. Thus, though there be hardly any tide visible on the open shores of the Mediterranean, there is a tide among the lagunes at Venice, at the head of the Adriatic.

The tide is also affected by any circumstance that alters the pressure of the atmosphere, as by winds, whether their cause be expansion or condensation of the atmospheric fluid.-But the whole phenomena of this very curious part of the effect of motion and gravitation, would demand many volumes for a complete detail: what has been stated, may enable any one to understand the general principles.

CONCLUSION.

THE FIXED STARS.

This is the most sublime page in the book of nature; but it is one upon which, though man can look with astonishment, and form conjectures which have no limit, it is one which he cannot read with the same certainty and satisfaction as he can the solar system.

The stars, including those which are invisible to the eye when unaided by a telescope, are about fifty thousand in number. They are distinguished by magnitudes, of which the first six are visible to the naked eye, and the others cannot be seen without telescopes. When we talk of the magnitude of a star, we must not, however, be betrayed by our common notions. The stars have no measurable magnitude whatever; they even seem smaller the more powerful the telescope through which they are viewed; and the apparent difference of magnitude, is difference of light, arising in all probability from difference of distance. We do not know the distance of any one of them; but we know that it cannot be less than twenty millions of millions of miles-a space over which the light of heaven itself could not travel in less than three years. We know that in this manner: the earth is, at the one part of its orbit, about two hundred millions of miles from where it is at the opposite point; but this two hundred millions of miles makes no alteration whatever in the situation of the brightest, and therefore probably, the nearest star; therefore that star must be so far away that, compared with its distance, two hundred millions of miles is too small for being discernible. If the star shifted but a single second, its distance would exceed the amazing extent that has been stated; but it does not shift even that little way, and therefore it must be much greater than this.

The mote in the sunbeam compared to the sun himself-compared to

the widest orbit of his attendant train, is gigantic when set against that sun or that orbit as compared with the nearest star. And what are their distances from each other? probably as immense as this. The modern belief is, that they are arranged into nebula or clusters; that our sun forms part of one of these, that the fifty thousand which have been counted, with myriads upon myriads that lie beyond, are all attended by their surrounding planets, and that thus, not the mere creatures of Almighty power; but the worlds, the systems of worlds, which God has made, and which God preserves, defy the powers of arithmetic; and turn philosophy to silent but grateful adoration; "All these glories are His, and yet He cares for me."