elevation with a suitable instrument, and apply certain corrections, to be hereafter explained. The problem of the longitude does not admit of so easy a solution. To determine accurately longitude at sea is a matter of the highest importance to commerce and navigation, a problem for whose solution maritime nations have in modern times offered large rewards. The safety of a vessel, its crew and cargo, depends on learning by some method its exact position on the surface of the ocean, where there are no permanent objects on our globe to mark its place; and it is only from the celestial sphere that it becomes possible to select fixed objects which may reveal to the mariner the dangers by which he is surrounded.

The latitude, as we have seen, is readily obtained; not so the longitude, which had, up to the time of Galileo,* been regarded as almost an impossible problem at sea. The great Florentine astronomer saw in the eclipses of the moons of Jupiter the means of solving this highly-important problem; and to this end he devoted many years to most diligent and careful observation of these eclipses, with a view to be able to predict their coming, months or even years in advance. We will now explain how these predicted eclipses of Jupiter's satellites, conjoined with their actual observation, may be employed in the determination of terrestrial longitude.

As the earth rotates on its axis with uniform velocity, the 360 degrees of the earth's equator are fairly represented by twenty-four hours of time. Thus an hour of time is equal to 15° of longitude, a minute of time is equal to 4' of longitude, a second of time is equal to 4" of longitude. The difference of longitude, then, of any two places on the earth's surface is nothing more than the difference of local time; for a mean time solar clock marks Oh. 00m. 00s. when the centre of an imaginary sun, moving with the mean or average velocity of the true sun, reaches the meridian of the place in question. A place west of the first one will have the

* Galileo flourished about 250 years ago, at Florence, in Tuscany.

centre of the mean sun on its meridian later by an amount of time equal to the exact difference of longitude. It is clear, then, that if any phenomenon, such as the sudden extinction of a fixed star, could be noted by two observers in different places, each will record the moment of disappearance in his own local time, and an inter-comparison of these records will give at once the difference of longitude between the two stations.

Suppose it were possible to predict that the bright star Vega, in the constellation of the Lyre, would suddenly disappear on the first day of January, 1870, at Oh. 00m. 00s. mean time at Greenwich, England, this fact being known and published, vessels at sea, on long voyages, in all parts of the globe, having the star above their horizon, by watching for this phenomenon, and by noting the moment of disappearance in their local time, would determine their longitude from Greenwich. All observers recording the disappearance before the predicted time would be in east longitude, while those recording the same phenomenon later than the predicted time would be in west longitude, and as many hours, minutes, and seconds west as was indicated by their local time.

Now, at sea, very simple methods, as we shall show hereafter, may be employed to obtain the local time; and thus, were it possible to predict a multitude of such phenomena as above recorded occurring every day or two for years in advance, seamen on long voyages, providing themselves with these predictions, would have the means of fixing their longitude as often as any one of those predicted phenomena could be observed.

The eclipses of the moons of Jupiter are precisely like the phenomenon of the sudden extinction of a star. As these moons shine only by reflected light, the moment they enter the shadow of their primary they vanish from the sight, or are, to all intents and purposes, tinguished; and as these eclipses are constantly recurring a very short intervals, Galileo saw at once the use to which they might be devoted

in the resolution of this great problem of terrestrial longitude.

Before they could be thus used, it became necessary to master completely their laws, so that the moment of eclipse might be accurately predicted years in advance. Though the Tuscan philosopher did not live long enough to perfect and apply his great discovery, his successors in modern times have fully carried out and applied what was SO admirably conceived and so carefully commenced.

An attentive examination of the luminosity of Jupiter's moons reveals the curious fact that it is variable, increasing and decreasing at regular intervals, equal to the periods of revolution in their orbits; whence it has been inferred by Sir William Herschel and others that each of these satellites rotates (like our moon) upon an axis in the exact time in which it revolves about the primary.

CHAPTER VIII.

SATURN, THE SEVENTH PLANET IN THE ORDER OF DISTANCE FROM THE SUN, SURROUNDED BY CONCENTRIC RINGS, AND ATTENDED BY EIGHT SATELLITES.

The most distant of the Old Planets.-Its Light faint, but steady.Synodical Revolution.-The Sidereal Revolution.-Advances in Telescopic Discovery.-Galileo announces Saturn to be Triple.-Huygens discovers the Ring.-Division of the Ring into Two.-Cassini announces the Outer Ring the brighter.-Multiple Division.-Shadow of the Planet on the Ring.-Belts and Spots.-Period of Rotation of the Planet and Ring. Disappearance of the Ring explained.-The Dusky Ring. SATELLITES OF SATURN.-By whom discovered.-Eight in number.Their Distances and Periods.-Saturn's Orbit the boundary of the Planetary System, as known to the Ancients.

WE now reach, in our outward journey from the sun, the most distant world known to the ancients, revolving in an orbit of vast magnitude, and in a period nearly thirty times. greater than that of our earth. Saturn, on account of h

immense distance, shines with a fainter light than either of the old planets, though still a conspicuous object among the fixed stars. Its light is remarkably steady, without the scintillations which distinguish the stars, and the brilliant glare which is shown by Venus and Jupiter. There is a yellowish or golden hue to this planet, which is not lost when seen through the most powerful telescopes.

Such is the planet Saturn, as known to the old astronomers, and as seen by the unaided vision. Its movement among the fixed stars is distinguished by the same phenomena which we have found to exist among all the planets. Being the most remote of all the old satellites of the sun, its stations are the best defined, its arc of retrogradation the shortest, and the period employed in this retrograde movement is longest. From observations made during opposition, and by trains of reasoning identical with those laid down in our examination of Jupiter, the periodic time and mean distance of Saturn are concluded.

Owing to the very slow motion of this planet in its orbit, the earth will pass between it and the sun, or bring it into opposition, in a little over 378 days; that is, Saturn and the earth starting from the same straight line, passing through the sun, the earth makes its revolution, comes up to the starting-point, and then overtakes Saturn in about twelve days and three-quarters. The earth's period must then be to that of Saturn as twelve days and three-quarters is to 378, or as one to thirty, roughly.

This determination is a matter of such simplicity, that any one, almost without instruments, may make the observations which give the data for the computation. The opposition is observed when Saturn is 180° from the sun; and we have only to count the days from one opposition to the next to obtain the synodical revolution.

Such were the few facts known to astronomy touching this distant orb prior to the discovery of the telescope. The

mense multiplication and extension of human vision ected by the invention and improvement of that instru

ment, is in no case more signally displayed than in the successive revelations which have been made in the physical constitution of Saturn, and the extraordinary appendages and scheme of dependent worlds now known to revolve around him.

In 1610, the year in which Galileo first applied the telescope to an examination of the celestial orbs-the year in which he announced the discovery of Jupiter's moonsan examination of Saturn resulted in the strange and anomalous discovery that his disc was not circular, like all the other planets, but elongated, as though two smaller planets overlaid a larger central one extending somewhat to the right and left of the centre. This remarkable figure Galileo announced to his astronomical contemporaries under the form of a puzzle produced by a transposition of the Latin sentence,—

"Altissimum planetam tergeminum observavi."

"I have observed the most distant of all the planets to be triple."

This mode of presenting the discovery was adopted by the Florentine astronomer to establish his priority, as many of his great discoveries were claimed by some of his opponents, while the truth of all was most obstinately disputed by others. It was urged, even in the case of Jupiter's moons, that these were mere illusions, the offspring of the heated imagination of the ambitious philosopher, and that other eyes could never verify these pretended discoveries. We can readily imagine what must have been the feelings of Galileo when, not many months after the discovery of the triple character of Saturn, he was compelled to acknowledge that, even as seen through his most powerful telescope, the planet was exactly circular, with an outline as sharp and perfect as that of Jupiter. He exclaims, "Can it be possible some demon has mocked me!" He did not live to explain this remarkable change; but he saw the triple form restored, and discovered these periodical transmutations of figure.

Fifty years later, in 1659, Huygens, with more powerful