at equal distances from one another, keep their relative places, and move together in a beautifully smooth manner. These spheres act as friction rollers in two directions at the four points of contact, in case any obstacle is opposed to their progressive motion by the admission of dirt, or by any change of figure of the wood that composes the rings of the dome, and of the gang-way. No groove is here made, but what the weight of the roof resting on the hard sphere occasions. The dome itself moves twice round for the balls once, and has, in this way, its friction diminished. The wood of this dome is covered by Wyatt's patent copper, one square foot of which weighs upwards of a pound; and the copper is so turned over the nails that fix it at the parts of junction, that not a single nail is seen in the whole dome. This covering is intended to render the dome more permanent than if it had been made of wood alone. At the observatory at Cambridge the dome is made chiefly of iron. In the figure a, a represents one of the two oblong doors that meet at the apex of the cone, and a piece of sheet-copper bent over the upper end of the door which shuts last, keeps the rain from entering at the place of junction. The two halves of the dome are united by brass rods passing through the door-cheeks of wainscot at a and a by means of nuts that screw upon their ends, which union allows the dome to be separated into two parts when there may be occasion to displace it. The wooden plate bb, which appears in a straight line, is a circular broad ring to which the covering wainscot boards are made fast above the eaves, and cc is a similar ring forming the wall-plate or gang-way on which the dome rests and revolves.

figure 92*.

Fig. 92* shows a small door that lies over the summit of the dome, and may be separately opened for zenith observations; the rod of metal with a ring at the lower end passing through it, serves to open and shut this door, and at the same time carries upon its upper end a large ball that falls back on the roof when the door is open, and keeps the door in a situation to be acted upon by the hook of a handle that is used for this purpose. The doors aa being curved, are made to open in two halves, the upper one being opened first, on account of its covering the end of the other; and the observer may open one or two doors as may best suit his purpose. The weight of this dome is such that a couple of wedges, inserted by a gentle blow between the rings bb and cc, will keep it in its situation under the influence of the strongest wind.

It may not be improper to remark, that in all observatories, and in every apartment where celestial observations are made, there should, if possible, be a uniform temperature; and consequently a fire should never be kept in such places, particularly when observations are intended to be made, as it would cause currents of air through the doors and other openings, which would be injurious to the accuracy of observations. When a window is opened in an ordinary apartment where a fire is kept, there is a current of heated air which rushes out at the top, and a current of

cold air which rushes in from below, producing agitations and undulations, which prevent even a good telescope from showing celestial objects distinct and well defined; and, I have no doubt, that many young observers have been disappointed in their views of celestial phenomena, from this circumstance, when viewing the heavenly bodies from heated rooms in cold winter evenings; as the aërial undulations before the telescope prevent distinct vision of such objects as the belts of Jupiter, the spots of Mars, and the rings of

Saturn.

L

CHAPTER IV.

ON ORRERIES OR PLANETARIUMS.

AN orrery is a machine for representing the order, the motions, the phases, and other phenomena of the planets. Although orreries and planetariums are not so much in use as they were half a century ago, yet as they tend to assist the conceptions of the astronomical tyro in regard to the motions, order, and positions of the bodies which compose the solar system, it may not be inexpedient shortly to describe the principles and construction of some of these machines.

The reason why the name Orrery was at first given to such machines, is said to have been owing to the following circumstance. Mr. Rowley, a mathematical-instrument-maker, having got one from Mr. George Graham, the original inventor, to be sent abroad with some of his own instruments, he copied it and made the first for the Earl of Orrery. Sir R. Steele, who knew nothing of Mr. Graham's machine-thinking to do justice to the first encourager, as well as to the inventor of such a curious instrument, called it an Orrery, and gave Mr. Rowley the praise due to Mr. Graham. The construction of such machines is not a modern invention. The hollow sphere of Archimedes was a piece of mechanism of this kind,

having been intended to exhibit the motions of the sun, the moon, and the five planets, according to the Ptolemaic system. The next orrery of which we have any account was that of Posidonius, who lived about 80 years before the Christian era, of which Cicero says, 'If any man should carry the sphere of Posidonius into Scythia or Britain, in every revolution of which the motions of the sun, moon and five planets, were the same as in the heavens, each day and night, who in those barbarous countries could doubt of its being finished-not to say actuated-by perfect reason?' The next machine of this kind, which history records, was constructed by the celebrated Boethius, the Christian Philosopher, about the year of Christ 510-of which it was said that it was a machine pregnant with the universe-a portable heavena compendium of all things.' After this period, we find no instances of such mechanism of any note till the 16th century, when science began to revive, and the arts to flourish. About this time the curious clock in Hampton Court Palace was constructed, which shows not only the hours of the day, but the motions of the sun and moon through all the signs of the zodiac, and other celestial phenomena. Another piece of mechanism of a similar kind is the clock in the cathedral of Strasburg, in which besides the clock part, is a celestial globe or sphere with the motions of the sun, moon, planets and the firmament of the fixed stars, which was finished in 1574.

Among the largest and most useful pieces of machinery of this kind, is the great sphere erected by Dr. Long in Pembroke Hall in Cambridge. This machine, which he called the Uranium, consists of a planetarium which exhibits the motion of the earth and the primary planets, the sun, and