it appears like a star of the 6th magnitude. It had been observed by Flamsead and Mayer; but was considered by them as a fixed star, and put down in catalogues as such. Long before the discovery of this planet, some disturbances and deviations in the motions of Jupiter and Saturn were observed by astronomers, which they could only account for on the supposition that there existed some planet still more distant from the sun than Saturn. But this can only be considered in the light of mere conjecture. For it is to the indefatigable industry and exertions of Dr. Herschel, that we are indebted for the discovery of this planet, and all the important particulars connected with its motion.

Uranus is at such an immense distance from the sun, that the light he receives from that luminary must be very small indeed; but this want is in a great measure supplied by six satellites which revolve round him, all of which were discovered by Dr. Herschel. The first and fourth he discovered in January 1787; and the other four in 1790 and 1794.

The orbits of all the satellites are nearly in the same plane, and almost at right angles to the orbit of Uranus; but what is rather extraordinary, the motion of these satellites is retrograde, or directly the reverse of the other planets and satellites.

The distances and periodic times of the satellites are as in the following table:

Eclipses of the satellites sometimes take place; but these can only be seen when Uranus is near his opposition. This happened in the year 1799, and also in 1818; but they can only be discerned by the most powerful telescope, and when the sky is perfectly clear.

THE PLANETS, AS SEEN THROUGH A TELESCOPE.

If eager still in Nature's book to pry,

Thou should'st the Astronomic tube apply;
And trace with careful eye the wide inane,
The Comet's blaze, and Planetary train :
Then shalt thou mark the various systems roll,
And learn the laws that regulate the whole.

CAREY.

Although many of the phenomena mentioned in the foregoing pages have been discovered by means of the telescope, yet this most valuable instrument has been the means of accomplishing many other splendid

discoveries, which could not be noticed, consistently with the plan of this work, at an earlier period.

The telescope was first turned to the heavens by the celebrated astronomer and philosopher, Galileo; hence it has been aptly enough styled the Galilean tube.*

By whose aid are seen

EUDOSIA.

The planetary phases, the bright cohort Of secondary worlds; and countless suns, Which, hid in the immensity of space, Ne'er visited the sight: from whom we learn The eclipse in time and quantity exact; And trace the parallax, that wondrous clue, By which the distance and the magnitude Of the celestial spheres are known on earth. No astronomer has ever done more to improve the telescope, or made more observations and discoveries in the heavens by means of this most amusing and useful instrument, than the late Dr. Herschel.t It may therefore be gratifying to hear the Doctor's observations respecting the requisites which this instrument ought to possess, in order to enable the observer to make accurate and minute observations on any of the celestial bodies.

In stating the result of a series of observations on the satellites of Uranus, or the Georgian planet, as the Dr. terms it, he says, "The great distance of this planet renders an attempt to investigate the movements of its satellites a very arduous undertaking; for their light, having to traverse a space of such vast extent before it can reach us, is so enfeebled, and their apparent diameter so diminished, that an instrument, to be prepared for viewing them, must be armed with the double power of magnifying and of penetrating into space." "The first of these properties," continues the Dr. seems not to be generally understood: the question how much a telescope magnifies admits of various answers. To resolve it properly, we ought, in all circumstances, to consider how far the magnifying power of a telescope is supported by an adequate quantity of light; for without it, even the highest power and distinctness cannot be efficient. This is abundantly confirmed when a ten-feet reflector is directed to the Georgian planet; for with none of its highest powers can we possibly ascertain even the existence of the satellites.

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"Since, then, it is absolutely necessary that the power of magnifying should be accompanied with a sufficient quantity of light to reach the satellites of this remote planet, it may be useful to cast an eye upon the action of a power which is become so essential. Its advantages and its inconveniences must equally be objects of consideration.

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A very material inconvenience is, that mirrors which must be large in order to grasp much light, must also be of a great focal length; and that in consequence of this we must submit to be encumbered with a large apparatus.

* See page 9 of the Supplement to this Work.

As he is better known by the little Dr. than Sir William Herschel, we retain the former.

"The forty-feet telescope having more light than the twenty-feet one, which I frequently use, it ought to be explained why I have not always used it in making my observations on the satellites of the Georgian planet. Of two reasons that may be assigned, the first relates to the apparatus and the nature of the instrument. The preparations for observing with it take up much time, which in fine is too precious to be wasted in mechanical arrangements. The temperature of the air for observations that must not be interrupted, is often too changeable to use an instrument that will not easily accommodate itself to the change; and since this telescope, besides the assistant at the clock and writing-desk, requires moreover the attendance of two workmen to execute the necessary movements, it cannot be convenient to have every thing prepared for occasional lucid intervals between flying clouds that may chance to occur; whereas in less than ten minutes the twenty-feet telescope may be properly adjusted and directed, so as to have the planet in the field

of view.

"In the next place I have to mention, that it has constantly been rule with me, not to observe with a larger instrument when a smaller would answer the intended purpose. To use a manageable apparatus saves not only time and trouble, but what is of still greater consequence, a smaller instrument may comparatively be carried to a more perfect degree of action than a larger one; because a mirror of less weight and diameter may be composed of a metal which will reflect more light than that of a larger one; it will also accommodate itself sooner to a change of temperature; and when it contracts tarnish, it may with less trouble be repolished."

After describing the effects of different kinds of eye-glasses, the Dr. states that the magnifying power by which the satellites of Uranus were discovered was only 157; and that he constantly used the same power in his sweeps of the heavens, and found it to be very effective for the discovery of faint nebulæ, and minute clusters of stars, but hardly sufficient to show the satellites steadily.

The higher powers of 2400, 3600, and 7200, were only employed to scrutinize the closest neighbourhood of the planet, in order to discover additional satellites; but from the appearance of the known satellites with these powers, they were found too indistinct to be used.*

It has already been remarked, that both Mercury and Venus exhibit all the various phases of the moon. This, however, cannot be perceived without the aid of a very good telescope. But with this assistance the eye can very distinctly discern all the variety of forms in these two planets, which can be so easily perceived in the moon by the naked eye. After their inferior conjunction, that is, when first seen in the morning, rising a little before the sun, they exhibit the form of a crescent. When they have attained their greatest western elongation, they appear nearly half illuminated. As they return towards the sun, their disc becomes more and more illuminated, till they reach their superior conjunction, when it is

The telescopes here mentioned are of the reflecting kind, the mirrors of which are metal, or rather an alloy of various metals. The great mirror of the 40-feet telescope is four feet in diameter, and weighs above 2000 pounds.

completely illuminated; but being on the opposite side of the sun with respect to the earth, they are lost in his rays. When they begin to be seen in the evening, their disc appears gibbous, or more than half; and when they have reached their greatest elongation, it again appears about half enlightened. In their return to the sun, the luminous disc continues to diminish till they arrive at their inferior conjunction, and then their dark side is completely turned towards the earth.

In the interval between the disappearing of these planets in the evening, and their re-appearing in the morning, they sometimes pass over the sun in the form of a dark spot. See page 25. This appearance would happen at every inferior conjunction of Mercury and Venus, and their passage would be right across his centre, if their orbits were co-incident with the ecliptic. But on account of both their orbits being inclined several degrees to the ecliptic, they are never seen to cross the sun's disc but when they are very near any of their nodes at the time of their inferior conjunctior, which is very seldom the case. At all other times they pass either above or below the disc of the sun. The transits of Venus succeed each other at the intervals of eight and one hundred years alternately.

The planet Mercury is so much immersed in the sun's rays, that few observations can be made upon him. It is, therefore, almost impossible to observe points of unequal splendor on his disc; and yet certain periodical inequalities have been observed in the horns of the disc. This circumstance has led some astronomers to conclude that Mercury has a revolution on his axis as well as the planets Venus, Mars, Jupiter, and Saturn, each of which has had its revolution on its axis determined by the same means; namely, the motion of certain spots, distinguished by the colour or intensity of their light from the other parts of the planetary disc.

The astronomer Shroeter, by continued observations on the horns of Venus, and attending to the variations in the appearance of some luminous points near the edges of the unenlighted parts, has not only ascertained that Venus has a revolution on its axis, but that its surface is diversified with mountains of very great height. The following figure, A, represents Venus according to Herschel, and B and Č according to Shroeter; they are, however, inverted.

ར་

The extreme difficulty, however, of seeing the spots on this planet even with the best telescopes, in our climate, prevents such observations from being so often repeated as could be wished. In point of brilliancy, Venus surpasses all the other planets. On some occasions it is so bright as to be seen in full day by the naked eye.

The gibbous appearance, and spheroidical figure of Mars, could never have been discovered without the assistance of very powerful telescopes; for, as we have already remarked, the true figure of this planet was unknown till the time of Dr. Herschel. But the industry and ingenuity of this celebrated astronomer, in constructing telescopes of the most powerful kind, and making observations with them, has not only ascertained the real figure of this planet, but also of almost all the other planets; and by the spots which he discovered near the poles of Mars, he has been enabled to settle the inclination of its axis to the plane of its equator, and consequently to ascertain its change of seasons. The following figures, A, B, C, and D, represent this planet as seen by Dr. Herschel, with his best telescopes :

From these discoveries it appears, that the analogy between Mars and the Earth is greater than between the Earth and any other planet of the solar system. Their diurnal motion is nearly the same; the obliquity of their respective ecliptics, on which the seasons depend, are not very different; and of all the superior planets, the distance of Mars from the Sun is by far the nearest alike to that of the Earth; nor is the length of its year very different from ours, when compared with the years of Jupiter, Saturn, and Uranus.

The telescopic appearance of the planet Jupiter having been pretty fully treated of at page 36, little more remains to be said respecting it. With a telescope of a very moderate power, the disc of Jupiter appears nearly as large as the Moon; and though the surface be diversified by regular and parallel belts, yet it appears much smoother than that of the Moon. The following figures, A and B, exhibit the appearance of this planet as seen by Herschel with his best telescope.