it has an area four times greater than that of the threefeet speculum. When finished, the speculum was placed in a square box, which is attached to the lower end of the tube, and by means of a door can be entered at pleasure. This box adds six feet to the length of the tube, which, like its predecessor, is of wood, hooped with iron like a barrel, and so wide that a tall man could walk through it without stooping. This huge black funnel is suspended between high and strong walls. It swings with a clear space of twelve feet on each side; and so far it can be drawn aside, giving half an hour before and after meridian. By means of a windlass, and a most skilful adjustment of chains and counterpoising weights, it can also be brought to the zenith, or turned fairly round from south to north. Enormous as are its dimensions, and although weighing altogether twelve tons, it seems to be about as easily moved as the other telescope; and it is as much in the mechanical contrivances for effecting this purpose as in anything else that the peculiar merit of the structure consists.

CHAPTER IV.

NEBULAR AND SIDEREAL SYSTEMS.

The Milky way. Comparative dimensions of the Solar and Sidereal Systems. Distances of the Fixed Stars. Classification of Stars according to their apparent Magnitudes. Distribution of the Stars. Gauging of the Heavens by Herschel. True form of the Milky Way. Clusters and Nebula. Forms and distribution of Nebulæ. Vastness of the Universe. Effect of the finite velocity of Light.

$75. In that portion of infinite space which is unveiled to the gaze of man lie clusters of countless suns, separated one from another by unimaginable intervals. Within one of these clusters, and probably nowise distinguished as to size or brightness from the other orbs, lies the sun around which our earth revolves. About this sun, at distances too small to be represented here, revolve the members of

the solar system. With this bright company are we environed day and night.

Fig. 1, plate I, represents the outline of a section of the cluster to which it is supposed our sun belongs. The section makes an angle of 35 degrees with the earth's equator, crossing it in 1241° and 3041° of right ascension. A celestial globe adjusted to the latitude of 55° north, and having a Ceti near the meridian, has the plane of this section pointed out by the horizon. It cuts the milky way at right angles on one side in its two branches which cross the constellation of the Eagle, and on the opposite side in the southern part of the Unicorn towards the Canis Major. The circle in the figure includes all the suns or stars ever visible to the naked eye. On all sides of the earth, taken together, from four to eight thousand (for the number is dif ferently estimated) may be seen. About two thousand may be seen by average eyes on an ordinary night in clear climates. In foggy island climates not more than nine hundred are visible at once. Inexperienced observers suppose the number much larger, partly because the sight is dazzled by their irregular distribution, and partly because as they diminish from stars of large size to those scarcely visible the imagination supposes others still smaller and invisible. The dot in the centre of the figure gives the position of our sun in the cluster. All stars beyond the circle, if they lie scattered in space are invisible to the unassisted eyes of the inhabitants of the earth. If they lie many in one direction, they present to the naked eye a milky, hazy appearance, and are called by the general name of nebulæ. This nebulous light belongs to distant groups of our cluster, and also to more remote clusters. Stars one hundred and eighty times the distance of Sirius are the most remote which appear even as nebulæ to the naked eye.

§ 76. In order better to conceive of the dimensions of the solar system, and of our cluster, and the intervals which lie between the clusters, let us make ourselves familiar with known and moderate distances, and advance from these to those which almost baffle the imagination.

The sun is a globe 383,000 miles in diameter. A hollow sphere with a radius of three thousand millions of miles.

includes all the members of the solar system yet discovered. If the swiftest race horse had begun to traverse this sphere at full speed at the birth of Moses, thirty-four centuries ago, he would not yet have accomplished one quarter part of his journey.

§ 77. Between the solar system and the stars lies a wide space traversed only by comets, and their appropriate field, if indeed they be not visitants from other spheres. The nearest fixed star, a Centauri, is twenty-one millions of millions of miles from the sun; 61 Cygni is fifty-six millions of millions distant from it. The distances of but few of the fixed stars have yet been ascertained. What we know of their distribution makes it probable that the stars of one cluster are on an average separated among themselves by distances as great as that between our sun and the nearest fixed stars.

The distances of those stars which have not yet been measured can only be inferred from their superior brightness. And here again for want of knowledge we must introduce another supposition. We must suppose that stars appear large merely in consequence of their proximity to us; and we must leave out of sight the differences which have lately been proved to exist in their actual size, or in the intrinsic brightness of their surfaces. As we know nothing of these particulars, and as they may vary in different stars in the ratio of many millions to one, we cannot be sure that we assign to any star its true distance. An arrangement of the stars in the order of their precise apparent brightness is much to be desired; but the variety of their color makes such an arrangement difficult. If they were catalogued according to the force of the whole impression made on the eye, we might obtain some knowledge about their intrinsic light-giving power, and might ascer tain the extent of the changes which take place in the light of some of them.

§ 78. At present the stars are loosely divided into classes according to their apparent size. All above a certain size are considered of the first magnitude; all less bright than these, and above a certain brightness, are of the second magnitude. Those decidedly inferior in bril

liancy form the third class, and so on down to the sixth and seventh magnitudes, which comprise the smallest stars visible to the naked eye in the clearest night. Beyond these the telescope reveals new orders, and as higher space-penetrating powers are used, new orders are added. Of course the layer in which a star first appears does not give us its position in space, it may be a very large star and lie farther off, or it may be a small one and lie in the layer in which it first appears to us. All we expect to learn is the comparative brightness of the stars as seen from the solar system.

The division into magnitudes is arbitrary, nor is it easy to determine where one magnitude ends and another begins, since all those stars which are included in one magnitude are by no means of the same size.

The light of Sirius, the brightest of the fixed stars, is about 324 times that of an average star of the sixth magnitude. As might be expected, the number of stars of each magnitude increases rapidly as we pass from the first to the lower magnitudes. There are from fifteen to twenty stars in the first class, and, unfortunately for us in the northern latitudes, the largest and most brilliant of these are not visible in our heavens. Of the second magnitudes there are fifty or sixty stars; of the third, 200; and in the first seven classes taken together, there are upwards of 2,000 in the northern hemisphere; in the milky way about 18,000,000; and in all the nebulæ, about 100,000,000 distinct stars are within reach of telescopic vision.

§ 79. The three or four brighter classes are distributed with tolerable equality throughout the heavens, but the smaller ones visible to the naked eye increase rapidly in number as we approach the borders of the milky way. The telescopic stars are crowded beyond imagination along that circle and the branch which it sends off, so that its whole light is composed of stars, whose average magnitude is not above the eleventh or twelfth. It was computed by Herschel that in one hour 50,000 passed through the field of his telescope in a zone 2° in breadth. This compression was partly owing to the vast numbers brought

within his line of vision in depth, and partly to the real crowding of the stars in the milky way.

If one

This unequal distribution of stars enables us to learn approximately the form of our cluster. Sir William Herschel determined it on the following principle. If you were in a crowd or immense building filled with people, you would judge your distance from the edges of the crowd or from the walls of the hall from the number of people seen in each direction. If you were in a wood and wished to determine its outline without leaving its interior, you might form a rude approximation to the true outline by taking your position in one spot and drawing imaginary lines in every direction to the edge of the wood. hundred trees were visible in one direction, you might assume for the line running thither a certain length, and proportion all your other lines to this, making them longer or shorter as more or fewer trees were visible in each direction. A bounding line passing through the termination of each of these lines would be not far from the true outline. A body which has extension in three directions, as our cluster, may be treated in the same manner as the wood. Herschel used the telescope as a sounding line, and inferred from its discoveries the hollows, protuberances, and in fact the shape of a great portion of our cluster. He made 700 observations to fix its form and dimensions.

In order to determine the comparative mean richness in stars of any two regions of the firmament, Herschel made úse of a telescope which magnified 187 times, and whose field embraced a circle of 15' diameter. This field included each time about the eight hundred and thirty thousandth part of the entire heavens. Towards the middle of the first of these regions he counted successively the number of stars included in ten fields contiguous or at least very near each other. He added these numbers and divided the sum by 10. The quotient was the mean richness of the region explored. The same operation, the same numerical calculation, gave him the mean richness of the second region. When the latter result was double, triple, or tenfold the former, he inferred that a stratum of it contained twice, three times, or tenfold as many stars as a stratum