by the same astronomer, but since that time no search has been able to detect it. The star is gone; whether never to return, it is impossible to say. A like disappearance has occurred with reference to the stars numbered 80 and 81, both of the fourth magnitude, in the same constellation. In May, 1828, Sir John Herschel missed the star numbered 42 in the constellation Virgo, which has never since been seen. ples might be multiplied, but it is unnecessary.

Exam

In these cases the stars have been lost entirely ;— no return has ever been marked; and but for the discovery of another class of remarkable objects among the stars, no return would probably ever have been suspected. If I could direct your attention to-night. to a brilliant star named Algol, in the head of Medusa, and bring a powerful telescope to aid in your examinations, this star, while you are watching it, might be seen to lose its splendor, and from its rank of the second magnitude to decline in brightness, until it would scarcely be visible to the naked eye. Having reached a certain limit, it would commence an increase, and by slow degrees resume its original splendor. This decrease and increase is actually accomplished in about eight hours. Having regained its usual light, it remains stationary for about two days and a half, and then repeats the changes already detailed; and thus have its periodical fluctuations continued since the date of its discovery, with the most astonishing regularity. The bright star marked Beta, in the constellation Lyra, is known to pass from the third to the fifth magnitude, and to regain its light in a period of six days and nine hours. These are called periodical stars and a sufficient number

have already been detected to present a progressively increasing series of periods from two days twenty hours up to four hundred and ninety-four days, and in one case even many years.

Here, again, are phenomena indicative of extraordinary activity in these remote regions of space.No explanation of these changes has yet been given in all respects satisfactory. Some have attributed them to the existence of dark spots on the stars, which, by rotation on an axis, periodically present themselves, and thus dim the lustre of the stars. Others think the changes are due to the revolution of large planets about the stars, which, by coming between the eye and the star, eclipse a portion of its light, while a third class conceive the fluctuations to arise, in some instances at least, from an orbitual motion of the stars in orbits of excessive elongation, and so located as to have their greater axes directed towards our system.

It will be seen that this theory may be readily extended so as to embrace the new stars already referred to, and even to account for those which have been lost from their places in the heavens. Here, however, we enter the confines of the uncertain. Centuries may roll away before the true explanation of these astonishing changes shall be given; but the mind is on the track, and with a steady and resistless movement is slowly pushing its investigations deeper and still deeper into the dark unknown.

While the phenomena of the new and lost stars and the fluctuations in the light of the variable ones gave undeniable evidence of constant change in what Aristotle was pleased to call the eternal and incor

ruptible heavens, Herschel's brilliant discovery of the orbitual motion of the double stars gave to the mind the opportunity of determining the nature of the law which sways the movements in these distant regions of space. It was natural, in the first efforts to compute the orbits of the double stars, to adopt the hypothesis that they attracted each other by the same law which prevails in the planetary system. Results did not disappoint expectation.-Gravitation, which Newton, in the outset of his great discovery, had boldly affirmed exerted its influence wherever matter existed or motion reigned, was extended, in the most absolute manner, to the region of the fixed stars There, at a distance from our own system almost inconceivable, suns and systems of suns, rising in orders of greater complexity, revolving with swift velocity, or with slow and majestic motion, bore testimony, ample and unequivocal, to the truth of the great law of universal gravitation.

Every particle of matter in the universe attracts every other particle of matter with a force which is proportioned directly to the mass, and which decreases as the square of the distance at which it operates increases. This is no longer a bold hypothesis. The double star marked Zeta, in the constellation Hercu les, has been subjected to the analysis of the computer. The elements of its orbit have been obtained and true to its predicted period, it has actually performed an entire revolution in a period of thirty-five years. The components of the star Eta, in the Northern Crown, revolve around their common centre in about forty-four years. Both of these pairs have completed an entire revolution since their discovery

Many others might be named, but my only object, at present, is to exhibit the evidence which shall remove all doubt as to the actual extension of the law of gravitation to the fixed stars.

Let it be remembered that this department of astronomy is yet in its infancy. Thousands of double stars have been detected, and every year adds hundreds to the list. Among these, a large proportion must prove to be binary systems, varying in their periods of revolution, from thirty years or less, up to many thousands, perhaps millions of years.

The association of two suns naturally suggests the possible union of a greater number, forming more complicated systems. This idea has been verified-a large number of triple systems has been discovered. In a few instances quadruple sets have been found, of which a remarkable example exists in the constellation of the Harp. Here was found four suns, arranged in pairs of two. The components of the first pair revolve around each other in about one thousand years; those of the second pair appear to require about double that period, while one pair revolves about the other in a period which, determined roughly from their distance, cannot fall much below a million of years! The evidence of the physical union of these four stars into one grand system rests, at present, on the ascertained fact that their proper motions are the same.

From quadruple systems we rise, by analogy, still higher, until we find hundreds, sometimes thousands of stars compacted together in so small a compass that their proximity cannot be the effect of accident. Look at the beautiful little cluster called the Pleiades

an ordinary eye may here see six or seven stars. One of very great power has been known to count four teen in this group, while the telescope increases the number to hundreds; and yet the space in which they are located might easily be covered by the moon.

Suppose an indifferent scattering of the stars through space, and compute the chances that such a number would fall together at any one point, and we shall find not one chance out of millions in favor of such an accident. We are therefore forced to the conclusion that here is a more magnificent order, one in which hundreds of suns, surrounded by their subordinate worlds, are all united by gravitation into one grand system. This is not a solitary example.— Many of these beautiful objects, comparatively close to our sun, are found in the heavens, leading the mind gradually up to the contemplation and examination of that mighty system of systems, that great cluster of clusters, the Milky Way, of which all these are but subordinate groupings,-vast in themselves, but when compared with the whole, mere units among the millions of which it is composed.

From what we have seen, it is impossible to avoid the conclusion that gravitation exerts its power among the myriads of shining orbs which strew the Milky Way. The innumerable suns which form this stupendous cluster must feel the reciprocal influence of each other, and nothing short of the centrifugal force arising from orbitual motion can balance this universal attractive power, and give to this grand system the great characteristic of stability.

Herschel succeeded, at least approximately, in sounding the profundities of the Milky Way, and fixed the