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.

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 heaven—a 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 the motion of the moon round the earth, all enclosed within a sphere. Upon the sphere, besides the principal circles of the celestial globe, the Zodiac is placed, of a breadth sufficient to contain the apparent path of the moon, with all the stars over which the moon can pass, also the ecliptic, and the heliocentric orbits of all the planets. The Earth in the planetarium has a moveable horizon, to which a large moveable brass circle within the sphere may be set coincident, representing the plane of the horizon continued to the starry heavens. The horizons being turned round sink below the stars on the east side, and make them appear to rise, and rise above the stars on the west side, and make them appear to set. On the other hand, the earth and the horizon being at rest, the sphere may be turned round to represent the apparent diurnal motion of the heavens. In order to complete his idea on a large scale, the Doctor erected a sphere of 18 feet diameter, in which above 30 persons might sit conveniently, the entrance to which is over the South Pole, by six steps. The frame of the sphere consists of a number of iron meridians, the northern ends of which are screwed to a large round plate of brass with a hole in the centre of it; through this hole, from a beam in the ceiling, comes the north pole, a round iron rod about three inches long, and which supports the upper part of the sphere, to its proper elevation for the latitude of Cambridge, so much of it as is invisible in England being cut off, and the lower or southern ends of the meridians terminate on, and are screwed down to a strong circle of oak 13 feet diameter, which, when the sphere is put in motion, runs upon large rollers of lignum vitæ, in the manner that the tops of some wind-mills turn round. Upon the iron meridians is fixed a zodiac of tin painted blue, on which the ecliptic and heliocentric orbits of the planets are drawn and the stars and constellations traced. The whole is turned round with a small winch, with as little labour as it takes to wind up a Jack, although the weight of the iron, tin, and the wooden circle is above a thousand pounds. This machine, though now somewhat neglected, may still be seen in Pembroke Hall, Cambridge, where I had an opportunity of inspecting it in November, 1839. The essential parts of the machine still remain nearly in the same state as when originally constructed in 1758.

The machine which I shall now describe is of a much smaller and less complex description than that which has been noticed above, and may be made for a comparatively small expense, while it exhibits, with sufficient accuracy, the motions, phases, and positions of all the primary planets, with the exception of the new planets, which cannot be accurately represented on account of their orbits crossing each other. In order to the construction of the Planetarium to which I allude, we must compare the proportion which the annual revolutions of the primary planets bear to that of the Earth. This proportion is expressed in the following table, in which the first column is the time of the Earth’s period in days; the second, that of the planets; and the third and fourth are numbers very nearly in the same proportion to each other.

365¼:88::83:20for Mercury.
365¼:224⅔::52:32for Venus.
365¼:687::40:75for Mars.
365¼:4332½::7:83for Jupiter.
365¼:10759⅓::5:148for Saturn.
365¼:30686::3:253for Uranus.