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velocity so great, that, in the case of y Virginis, an angle of 68° may be described in a single year. ☛ Coronæ will also attain its perihelion about 1835. Sir John Herschel, Sir James South, and Professor Struve of Dorpat, have increased Sir William Herschel's original catalogue to more than 3000, of which thirty or forty are known to form revolving or binary systems, and Mr. Dunlop has formed a catalogue of 253 double stars in the southern hemisphere. The motion of Mercury is more rapid than that of any other planet, being at the rate of 107000 miles in an hour; the perihelion velocity of the comet of 1680 was no less than 880000 miles an hour; but if the two stars of Ursæ be as remote from one another as the nearest fixed star is from the sun, the velocity of the revolving stars must exceed imagination. The discovery of the elliptical motion of the double stars excites the highest interest, since it shows that gravitation is not peculiar to our system of planets, but that systems of suns in the far distant regions of the universe are also obedient to its laws.
Possibly, among the multitudes of small stars, whether double or insulated, some may be found near enough to exhibit distinct parallactic motions, arising from the revolution of the earth in its orbit. Of two stars apparently in close approxi
mation, one may be far behind the other in space. These may seem near to one another when viewed from the earth in one part of its orbit, but may separate widely when seen from the earth in another position, just as two terrestrial objects appear to be one when viewed in the same straight line, but separate as the observer changes his position. In this case the stars would not have real, but only apparent motion. One of them would seem to oscillate annually to and fro in a straight line on each side of the other—a motion which could not be mistaken for that of a binary system, where one star describes an ellipse about the other. Such parallax does not yet appear to have been made out, so that the actual distance of the stars is still a matter of conjecture.
The double stars are of various hues, but most frequently exhibit the contrasted colours. The large star is generally yellow, orange, or red; and the small star blue, purple, or green. Sometimes a white star is combined with a blue or purple, and more rarely a red and white are united. In many cases, these appearances are due to the influences of contrast on our judgment of colours. For example, in observing a double star, where the large one is a full ruby-red or almost blood-colour, and the small one a fine green, the latter loses its colour when the former is hid by the cross wires
of the telescope. But there are a vast number of instances where the colours are too strongly marked to be merely imaginary. Sir John Herschel observes in one of his papers in the Philosophical Transactions, as a very remarkable fact, that, although red stars are common enough, no example of an insulated blue, green, or purple one has yet been produced.
Besides the revolutions about one another, some of the binary systems are carried forward in space by a motion common to both stars, towards some unknown point in the firmament. The two stars of 61 Cygni, which are nearly equal, and have remained at the distance of about 15" from each other for fifty years, have changed their place in the heavens during that period, by a motion which for ages must appear uniform and rectilinear because, even if the path be curved, so small a portion of it must be sensibly a straight line to us. Multitudes of the single stars also have proper motions, yet so minute that that of μ Cassiopeiæ, which is only 3" 74 annually, is the greatest yet observed; but the enormous distances of the stars make motions appear small to us which are in reality very great. Sir William Herschel conceived that, among many irregularities, the motions of the stars have a general tendency towards a point diametrically opposite to that occupied by the star
Herculis, which he attributed to a motion of the solar system in the contrary direction. Should this really be the case, the stars, from the effects of perspective alone, would seem to diverge in the direction to which we are tending, and would apparently converge in the space we leave, and there would be a regularity in these apparent motions which would in time be detected; but if the solar system and the whole of the stars visible to us be carried forward in space by a motion common to all, like ships drifting in a current, it would be impossible for us, who move with the rest, to ascertain its direction. There can be no doubt of the progressive motion of the sun and many of the stars, but sidereal astronomy is not far enough advanced to determine what relations these bear to one another.
The stars are scattered very irregularly over the firmament. In some places they are crowded together, in others thinly dispersed. A few groups more closely condensed form very beautiful objects even to the naked eye, of which the Pleiades and the constellation Coma Berenices are the most striking examples; but the greater number of these clusters of stars appear to unassisted vision like thin white clouds or vapours: such is the milky way, which, as Sir William Herschel has proved, derives its brightness from the diffused
light of the myriads of stars that form it. Most of them are extremely small on account of their enormous distances, and they are so numerous that, according to his estimation, no fewer than 50000 passed through the field of his telescope in the course of one hour in a zone 2° broad. This singular portion of the heavens, constituting part of our firmament, consists of an extensive stratum of stars, whose thickness is small compared with its length and breadth; the earth is placed about midway between its two surfaces, near the point where it diverges into two branches. Many clusters of stars appear like white clouds or round comets without tails, either to unassisted vision or with ordinary telescopes; but with powerful instruments Sir John Herschel describes them as conveying the idea of a globular space filled full of stars insulated in the heavens, and constituting a family or society apart from the rest, subject only to its own internal laws. To attempt to count the stars in one of these globular clusters,
he says, would be a vain task,-that they are not to be reckoned by hundreds, and, on a rough computation, it appears that many clusters of this description must contain ten or twenty thousand stars compacted and wedged together in a round space whose area is not more than a tenth part of that covered by the moon; so that its centre,