astonishment I perceived the same satellite outside the disk. It was in the same position as to being above a line with the apparent lower belt, when it remained distinctly visible for at least four minutes, and then suddenly vanished." The same appearance was noticed by Mr. Maclear of Biggleswade, and Dr. Pearson of South Kilworth, three observers at distant stations, with telescopes of different apertures, all positive as to the extraordinary deviation from rule. It could not, therefore, be an error of observation, or an illusion of vision, but was probably caused by some real, though transient, change in the deflection of light, of a purely incidental nature.

One instance is on record of Jupiter appearing deserted by his guards, all the satellites being behind the body of the planet, or immersed in his shadow. This was witnessed by Molyneaux, November 2, 1681. Such a spectacle has never been seen since; and supposing present arrangements to be maintained, the conjunction cannot occur again under an interval of ages which completely baffles our conceptions.

3. There is another obligation which we owe to these attendants of Jupiter, and not the least important, the discovery by their means of the progressive motion and appreciable velocity of light, which had before been considered instantaneous in its transmission. This was the achievement of the eminent Danish astronomer Olaus Roemer, in the year 1674; and he was led to it by observation of the eclipses of the satellites. Wishing to determine the exact period of the revolution of one of these bodies, he began to observe its successive eclipses, and to notice the time between them, as the means of ascertaining its velocity and motion. To attain the greatest possible precision, he continued his

investigations through successiveintervals. But Roemer had not done this many weeks before he was conscious that the time of the eclipse was a little later than it ought to be, according to computation founded upon previous observations; and it gradually became still later, till, at the expiration of six months, it was sixteen minutes behind the time computed from his first reckonings. After this period, continuing his watchings, the eclipse was found to be occurring gradually not so late as before, till at the close of another six months, the whole difference of sixteen minutes between the computed and observed times was done away with.

In explanation, the happy thought struck Roemer, that the observed time of an eclipse is never in any case the true time, but an interval afterwards, which light requires to reach the eye of the terrestrial spectator; and that the phenomenon of the eclipse gradually becoming later, and not so late, was simply occasioned by his own removal with the earth in its orbit to a varying distance from the system of Jupiter. Thus let A B C D represent the earth in different positions of its orbit; and J Jupiter surrounded by his satellites. It is obvious that when at A the earth is nearer the great planet by the whole. diameter of its orbit, than when at c. Now this change of distance would make no difference in the observed time of an eclipse if light were propagated instantaneously; but as there is the difference of sixteen minutes, light requires that time to travel over the space, amounting to 184 millions of

B

O

A

miles, which gives it a velocity of 185,000 miles a second.

The rate at which light travels is the swiftest movement in nature with which we are acquainted; and few greater feats have been performed than to measure the motions of an element that literally leaps over the gulf of nearly two hundred thousand miles while the pendulum inakes an oscillation. It is estimated that the time consumed by the electric current in traversing the distance between Greenwich and the Strand, say seven miles, is the зth part of a second; and experimental inquiry on some of the long telegraphic lines gives it about the same velocity. But this is scarcely one-ninth the rapidity of light. Yet it is a striking and obvious conclusion from the progressive motion of light -so vast is the scale of the universe-that we have no certain proof of the present existence of the sun or a star. If the orb of day were shorn of radiance at a given moment, we should not be conscious of the change till upwards of eight minutes after the catastrophe, as his expiring rays would not reach us under that interval. If Jupiter were to perish, announcement would not be made to us of the event under forty minutes; if Saturn, it would be an hour and a quarter; and if Uranus, two hours and a half. But there are stars so vastly remote, that with all its astonishing speed, the light that reveals them at the present instant only tells the tale of their existence, not so much in days that are gone, as in ages past, while the orbs themselves may have been extinguished for centuries by the power which lit them up in the awful distance.

Effect of Change of Distance-Telescopic Aid-Old Opinions of SaturnFirst Observations of Galileo-Three Bodies apparent-Changed Appearances-Discoveries of Huyghens-Annular Appendage announced -Change of Aspect Explained-Two Rings Observed-Discoveries of Cassini-Determinations of Sir W. Herschel-Position of Saturn in the Rings-Recent Observations-Bond and Lassel-Satellites of SaturnMagnitude of the Planet-Dimensions of the System-Thickness of the Rings-Solar Light and Heat.

AN object remarkable in itself, yet scarcely to be discerned in the haze of the remote horizon, commands no admiration, and excites no interest, unless we know beforehand what it is. But whether this knowledge be possessed or not, its true character is surely revealed on a near approach; and frequently the indistinct and

insignificant speck discloses stately proportions and a grand architectural character. It may be a castle of the olden time, with towers, turrets, and battlements, once inhabited by a baron bold; or a mansion of the Tudor age, with halls, corridors, galleries, oriel windows, tennis-court, and all the appurtenances deemed necessary by power, pride, or opulence. From the moment that this discovery is made, though the edifice is never approached again, and is only seen afar off as a puny thing, we think not of it as it appears in the distant landscape, but associate with it ideas in harmony with its real dimensions and actual details. The speck has for ever ceased to be one in our minds. It is a grim castle, or a noble mansion. Now precisely analogous is the effect which the telescope has produced with reference to the orbs of the universe. Though the interval between us and them remains really the same, yet it has been practically abridged by the instrument, for its optical power is equivalent to a corresponding lessening of the distance. Accordingly, since it was applied to celestial observation, a magic change has been wrought in human conceptions of the bodies in our system, as though a bridge had been partially thrown over the great gulf of space, which has brought us millions of miles nearer to their orbits; and we no longer think of them as they appear to the unassisted vision, but as exhibited by instrumental

means.

Among the corrections offered to thought by this practical approximation, perhaps the most striking is the change of ideas with reference to the planet Saturn, the sixth in order from the sun. It was for ages viewed as having no special claims to notice, and merely regarded as a dull, dreary, malignant star, with