Galaxy and nebulæ.

"It is very probable that the great stratum called the milky way, is that in which the sun is placed, though perhaps not in the very centre of its thickness. We gather this from the appearance of the galaxy, which seems to encompass the whole heavens, as it certainly must do if the sun is within the same: for, suppose a number of stars arranged between two parallel planes, indefinitely extended every way, but at a given considerable distance from one another, and calling this a sidereal stratum, an eye placed somewhere within it, will see all the stars in the direction of the planes of the stratum projected into a great circle, which will appear lucid, on account of the accumulation of the stars, while the rest of the heavens, at the sides, will only seem to be scattered over with constellations, more or less crowded, according to the distance of the planes, or number of stars contained in the thickness or sides of the stratum."

The nebula have been divided into three kinds. The first kind comprises those which consist of a great number of stars crowded together, and which are seen to be distinct through a telescope. Among these, is the famous nebula of Cancer, or the præsepe cancri, forming a collection of 25 or 30 stars, and many similar groups in various parts of the heavens.-The second kind consist of one or more stars, surrounded by a whitish spot, through which they seem to shine. There are several of this sort, but one of the most remarkable is that in Orion, which, through a telescope appears as a whitish spot nearly triangular: it contains seven stars, one of which is itself surrounded by a small cloud brighter than the rest of the spot. -Nebula of the third kind are white spots in which no stars are seen when viewed with a telescope.-Fourteen of these have been observed in the austral hemisphere, among which the celebrated spots, near the south pole, called by sailors the Magellanic clouds, hold the first rank.-Dr. Herschel has given catalogues of 2000 nebula and clusters of stars discovered by himself.

We will now, says the Doctor, in one of his papers on this subject, retreat to our own retired situation in one of the planets, attending a star in the great combination, with numberless others; and in order to investigate what will be the appearances from this contracted situation, let us begin with. the naked eye. The stars of the first magnitude being in all probability the nearest, will furnish us with a step to begin our scale; setting off therefore with the distance of Sirius or Arcturus, for instance, as unity, we will at present suppose, that those of the second magnitude are at double, and those of the

Galaxy. Power of Herschel's telescope.

third at treble the distance, and so forth. Taking it then, for granted, that a star of the seventh magnitude is about seven times as far from us as one of the first, it follows that an observer, who is enclosed in a globular cluster of stars, and not far from the centre, will never be able, with the naked eye, to see the end of it for since, according to the above estimations, he can only extend his view about seven times the distance of Sirius, it cannot be expected that his eyes should reach the borders of a cluster, which has perhaps fifty stars in depth every where around him. The whole universe, therefore, to him, will be comprised in a set of constellations, richly ornamented with scattered stars of all sizes. Or if the united brightness of a neighbouring cluster of stars, should, in a remarkably clear night, reach his sight, it will put on the appearance of a small, faint, nebulous cloud, not to be perceived without the greatest attention. Allowing him the use of a common telescope, he begins to suspect that all the milkiness of the bright path which surrounds the sphere may be owing to stars. By increasing his power of vision, he becomes certain that the milky-way is, indeed, no other than a collection of very small stars, and the nebulæ nothing but clusters of stars.

Dr. Herschel then solves a general problem for computing the length of the visual ray: that of the telescope which he uses, will reach to stars 497 times the distance of Sirius. Now according to his reasoning, Sirius cannot be nearer than 100,000 X194,000,000 of miles, therefore his telescope will, at least reach to 100,000×194,000,000X497 miles. And he observes, that in the most crowded part of the milky way, he has had fields of view that contained no less than 588 stars, and these were continued for many minutes; so that, in a quarter of an hour, he has seen 116,000 stars pass through the field of view of a telescope of only 15' aperture; and at another time, in 41 minutes, he saw 258,000 stars pass through the field of his telescope. Every improvement in his telescopes has discovered stars not seen before, so that there appears no bounds to their number, or to the extent of the universe.

The sun, like many other stars, has probably a progressive motion directed towards the constellation Hercules, carrying all its attendant planets along with it; and with respect to this motion, Dr. Herschel observes, that the apparent proper motions of 44 stars out of 56, are nearly in the direction which would be the result of such a real motion of the solar system; and that the bright stars Arcturus and Sirius, which are proba bly the nearest to us, have, as they ought, according to this theory, the greatest apparent motions. Again, the star Castor,

Fixed stars.-Concluding observations.

appears when viewed with a telescope, to consist of two stars, of nearly equal magnitude; and though they have an apparent motion, they have never been found to change their distance with respect to one another a single second, a circumstance easily understood, if both their apparent motions are supposed to arise from the real motion of the sun.

In the infancy of science, when the distances of the sun and planets were unknown, man, unable to draw any just conclusion respecting objects so much beyond his reach, never doubted that he was the only being of his kind in the universe, or that the earth was the most consequential aggregate of matter in existence. Pleased with what exalts himself, it was not without many a struggle, and by slow degrees, that he withdrew from the dominion of so contemptible a belief. At length, when the evidence, continually accumulating, triumphed over all contradiction; when the magnificence of the planetary orbs became perfectly evident; when it was proved that they certainly experienced the regular return of day and night, spring, summer, autumn, and winter; it became difficult to deny the plurality of inhabited worlds, with any hope of rationally explaining the design of that exertion of Creative Power which called those orbs into existence, and subjected them to the same laws as the earth; and man was constrained to admit, that a grain of sand was not more completely an atom to the earth, than the earth itself was an atom to the solar system. These ideas had scarcely obtained general acceptance, when it was found that the distances of all the fixed stars is immeasurably great; then it became obvious that they shine by their own light; that they are each of them comparable in real splendour and magnitude to the sun; that they are as distant from each other as any one of them must be from us; that therefore, instead of being merely as ornaments of the sky, the most reasonable conclusions of analogy inculcated, that they were the centres of so many systems, dispersed throughout the universe, and dispensing light and heat to planetary worlds around them; and the use of the telescope, at the same time that it imparted the power of counting millions of stars which the naked eye never beheld, yielded a convincing evidence, that millions of millions exist, at distances too great for them to be separately distinguished. Man had now advanced another step, but not in proving his own importance. On the contrary, from the insignificance of the earth to the whole solar system, he now advanced to the perception of the insignificance of the whole solar system, to the remaining immensity of creation; so that the total annihilation of the system that appears so vast to him, would be like the abstraction of nothing from the uni

Gravitation the cause of the tides.

verse. On the wings of conjecture, indeed, but still with analogy by our side, we may take a still more eminent if not expansive view. If there are reasons to believe that every orb of the solar system is inhabited, there are reasons scarcely less strong for believing that every star, and every orb attending every star, is created and exists only for the same great purpose. What incommunicable feelings do these views excite in the thinking mind! Perhaps all the stars that we can discover, and all, of the existence of which we can obtain any evidence, form a very finite part of the total number; yet, is it possible by any other means, to obtain so admirable a view of Infinite Power and Infinite Wisdom, as may be derived from the contemplation of the attributes of that Being who has created, and maintains, even that portion which we can actually discern of the stupendous fabric of Universal Nature?

OF THE TIDES.

Having now ranged over the mighty field of astronomical research, we must next proceed to consider a variety of phenomena, resulting from the general laws of nature, which are particularly and immediately interesting to us as inhabitants of the earth, but which would have too much interrupted our general view, if we had entered sufficiently into the details of them in the course of our progress. Here then we shall in the first place advert to those remarkable fluctuations of the ocean called Tides.

The apparent connexion subsisting between the movements of the ocean and those of the moon, has been observed from the earliest periods of antiquity; but it was Kepler who first asserted that the moon's attraction was the real cause: "If,” say he, "the earth ceased to attract its waters towards itself, all the water in the ocean would rise and flow to the moon. The sphere of the moon's attraction extends to our earth, and draws up the water." Sir Isaac Newton afterwards demonstrated the consonance of the cause assigned by Kepler, with his theory of universal gravitation, and explained at the same time the cause of the tides on the side of the earth opposite to the moon, and from his time to the present, not a doubt has been entertained on the subject.

The principal phenomena of the tides are as follows:

1. The sea is observed to flow for about six hours from south to north, gradually swelling; after this it seems to rest for a quarter of an hour; and then to ebb or retire back again from north to south, for six hours more. Then after another pause of about a quarter of an hour, the sea again begins to flow; and so on alternately.

General phenomena of the tides.

2. The time of a flux and reflux, is on an average about 12 hours 25 minutes, and twice this time, or 24 hours 50 minutes, is the period of a lunar day, or the time between the moon's passing a meridian, and coming to the same point of it again. So that the sea flows as often as the moon passes the meridian, that is, as well when she comes to the arch above the horizon, as when she comes to that below the horizon; and ebbs as often as the moon passes the horizon, both on the eastern and western side.

3. The elevation of the waters on that side of the earth immediately under the moon, somewhat exceeds the elevation of the opposite side, and in all cases the elevation diminishes from the equator to the poles.

4. The sun raises and depresses the sea twice every day, in the same manner as the moon; but the solar influence in this respect is less than that of the moon, in the proportion of 1 to 3.

5. The tides which depend on the actions of the sun and moon, are not distinguished but compounded; and thus they form, to appearance, one united tide, which, increasing and decreasing, produces Neap and Spring tides.

6. In the syzygies, that is, when the moon is either new or full, the action of both luminaries concur, and the tides are highest; but they are least in the quadratures, or when the lines of their action are 90 degrees apart, for where the water is elevated by the moon, it is depressed by the sun, and vice versa. Therefore while the moon passes from the syzygy to the quadrature, the daily elevations are continually diminished: on the contrary, they are increased, while the moon passes from the quadrature to the syzygy. At the new moon, also, the tides are the greatest, because the sun and moon are not only in the same line but on the same side of the earth; and at this time the tides of the same day are more different than those at full

moon.

7. The greatest elevations and depressions take place on the second or third day after the new or full moon; and they are the greater the nearer the luminaries are to the plane of the equator; they are therefore greatest in the syzygies at the time of the equinoxes.

8. The actions of the sun and moon are greater, the nearer these bodies are to the earth; and the greatest tides happen when the sun is a little to the south of the equator; but this does not happen regularly every year, because some variation may arise from the situation of the moon's orbit, and the distance of the syzygy from the equinox.