and at all events this experiment furnishes the means of determining the absolute value. Thus, while the circle furnishes the means of measuring the scale, the scale furnishes in return the means of measuring the subdivisions of the circle. These amount only to 360, and may be reduced even to the fifth part of this number, should practice prove this reduction desirable. This small number of divisions can rapidly be read up with a scale of invariable length, and by performing this reading at temperatures widely different, a correction for temperature may be determined with great exactness. In the old circle, as there are no less than 10,000 divisions, and as there exists no permanent scale for the reading of these divisions, it becomes almost impossible to learn their actual values and to tabulate their errors; hence astronomers have been compelled to rely to a great extent upon the assumed accuracy of the subdivisions of their circles, as received from the hands of the manufacturer.

By a combination of the electro-magnetic method with. the new method of measuring north polar distances, a very simple, convenient, and accurate instrument is obtained for recording the places of the stars or other heavenly bodies with great rapidity and exactitude, rendering it possible to construct, in a comparatively short time, a very extended and exact catalogue of the places of all the fixed stars, clearly visible, with any optical power.

We have thus presented a rapid sketch of the old and new methods of fixing the elements for the determination of the heavenly bodies: it only remains in this connection to speak of the optical power of the telescope.

These instruments are divided into two great classes, called reflecting and refracting telescopes. In the reflecting telescopes the rays of light from the external object, passing down the tube of the telescope, fall upon a metallic mirror or speculum, whose surface, perfectly polished, has the figure of a paraboloid of revolution. Being reflected by this surface, the rays of light are concentrated at a certain point, called the focus, where an intensely luminous image of the object

is formed. This image is then examined by a magnifying glass or eye-piece, and its dimensions expanded to any required degree.

In the refracting telescope the light falls upon what is called the object glass, a powerful lens, which concentrates, by refraction, the rays of light which pass through it, thus forming an image of the object at the focal point. This image is then examined, as in the reflecting telescope, by eye-pieces having different magnifying powers. Hitherto it has been found impracticable to construct object glasses of any very considerable diameter, the largest of these glasses in use not exceeding sixteen to twenty inches in diameter. These narrow limits do not exist, however, in the construction of the metallic specula which belong to the reflecting telescope; and hence we find gigantic instruments have been constructed by different observers, one of which, now in use by Lord Ross, has a speculum of no less than six feet in diameter, with a focal length of fifty-two feet. Such immense instruments, requiring ponderous machinery for their management, are not well adapted for that kind of observation which has for its object to determine the places of the heavenly bodies. Their use has been rather confined to examinations of the planets, double stars, clusters, and nebulæ, demanding a large amount of light rather than a perfect definition or exactitude in measurement. It is true, that in the hands of Lassell, of Liverpool, we find the reflecting telescope performing admirably in the routine. work of an observatory. But these instruments are com paratively small, their dimensions not much excceding those of the largest refractors.

There are two qualities which distinguish the telescope, the space-penetrating power and the power of definition. The first of these depends exclusively upon the amount of light received and refracted, or reflected to the focus, and thus forming the image. In case all the light falling upon the object-glass or speculum could be concentrated in the formation of the image, then the space-penetrating power of

telescopes would be exactly proportioned to the diameters. of their apertures; and we can compare then, readily, the space-penetrating power of different instruments, not only among themselves, but directly with the space-penetrating power of the human eye. The diameter of the pupil of

the eye determines the amount of light which can enter and form the image, just as the diameter of an object-glass in a telescope determines the amount of light which in that instrument forms the focal image: hence, if we desire to know how many times deeper a telescope can penetrate space than the eye, we have only to learn how many times the area of the object-glass exceeds that of the pupil of the eye. We shall have occasion hereafter to employ this principle when we come to examine the relative distances to which the nebulæ and clusters are sunk in space.

We have only spoken of the mounting of the transit, with its attached circle, for reading north polar distances. This instrument revolves only, as we have seen, in the plane of the meridian, and of course no object can be seen with the transit except when in the act of passing the meridian-line.

A telescope mounted in such a manner that it can be directed to any point of the heavens, is called an extrameridunal instrument, and of these the equatorial is the most used, and is the best adapted for all observations off the meridian. The tube of the telescope is carried by a heavy metallic casting, very firm and strong, which is made fast to a metallic cylinder, through which passes a steel axis, called the equatorial axis. The metallic cylinder is also screw-bolted to the extremity of a heavy steel axis, so placed on its supports as to lie parallel to the earth's axis. These supports rest on heavy metallic plates, bolted to a massive stone pier, called the "foot of the instrument," which, in turn, is placed on the top of a heavy pier of masonry, resting on a rock-foundation, or something equally solid, and entirely disconnected from the building.

The instrument is so counterpoised in all its many parts as to be readily moved either on its polar or equatorial axis,

and may thus be directed to any point of the celestial sphere. To enable the observer to follow the object under examination, these telescopes are usually furnished with a species of clock-work, which causes the instrument to revolve round its polar axis with a velocity equal to that of the earth's rotation, causing it to follow a heavenly body, and to hold it steady in the field of view for any required period of time.

Without extending further our notice of the instruments employed in reaching the data required in astronomical investigation, we will now return to our examination of the bodies which compose the sun's retinue, and shall proceed in our plan, preserving the order of distance from the sun.

The interruption which was made after closing the discussion of the system of Saturn, to introduce to the student the laws of motion and gravitation, and the instruments employed in astronomical measures, was necessary to a full comprehension of the extraordinary investigations which are now to follow. We are hereafter to treat the planets and their satellites as ponderable bodies, mutually affecting each other, and all subjected to the dominion of the laws of motion and gravitation.

CHAPTER XII.

URANUS, THE EIGHTH PLANET IN THE ORDER OF
DISTANCE FROM THE SUN.

Accidentally discovered by Sir William Herschel.-Announced as a Comet. -Its Orbit proved it to be a Superior Planet. -The Elements of its Orbit obtained.-Arc of Retrogradation.-Period of Revolution.-Figure of the Planet.-Inclination of its Orbit.-Six Satellites announced by the elder Herschel.-Four of these now recognized.-Their Orbital Planes and Directions of Revolution Anomalous. Efforts made to Tabulate the Places of Uranus unsuccessful.-This leads to the Discovery of a New Exterior Planet.

Ir was remarked at the close of our investigation of the Saturnian system, that this planet inclosed by its orbit al

the objects belonging to the solar system which were known to the ancients, and whose phenomena, as observed and recorded in all time, furnished the data for the discovery of Kepler's laws and the law of universal gravitation, as finally revealed by Newton. While many of the modern astronomers, from an examination of the inter-planetary spaces, had ventured to suggest the probable existence of a large planet revolving in an orbit intermediate between those of Mars and Jupiter, no one had ventured to predict the possible discovery of planets lying exterior to the mighty orbit of Saturn. From the very dawn of astronomy this planet had held the position of sentinel on the outposts of the planetary system, and many strong minds had long entertained the opinion that no other bodies existed exterior to the orbit of Saturn, forming a part of the scheme of worlds revolving around the sun. Such, indeed, was the prevalence of this opinion, that when, in 1781, Sir William Herschel, in a course of systematic exploration of the heavens, discovered an object having a well-defined planetary disc, and whose movement among the fixed stars became measurable, even at the end of a few hours, he did not even suspect this new object to be a planet, but announced to the world that he had discovered a most extraordinary comet, without any of the usual haziness which attends these bodies, but presenting a clear and well-defined planetary disc.

This newly-discovered object soon attracted universal attention. It was observed at the royal observatory at Greenwich, and the then astronomer royal, Dr. Maskelyne, was the first to suspect its planetary character. Efforts were made by several computers to give to the new comet, as it was called, a parabolic orbit; this, however, was found to be impossible; and it was very soon found that the newlydiscovered object was revolving around the sun in an orbit nearly circular in form, lying in a plane, nearly coincident with the ecliptic, and completing its mighty revolution in a period of no less than eighty-two years. It must be remembered that these extraordinary discoveries and announce