ORRERY. An astronomical instrument for exhibiting the several motions of the hea venly bodies. The first machine of this kind was constructed by Mr. Graham, but it derives its name from the Earl of Orrery, for whom one was made by Mr. Rowley ; and Sir Rich ard Steele supposing this to be the first ever constructed, he gave it the above name in honour of the earl. Orreries are constructed so as to be more or less complete. That given in the accompanying figure has all the planets represented upon it ; that is to say, the sun, placed in the centre of the solar system ; next to the sun is the orbit of Mercury; and next to that, the orbit of Venus. Next to the orbit of Venus, is the orbit of our earth, represented by a silver plate, on which the signs of the zodiac, the degrees of the ecliptic, and the days of each month aro drawn ; and the earth is represented by an ivory ball placed upon an axis, so as to make an angle with the plane of the horizon of 60 degrees and a half. About the ivory ball there is a silver circle, which is placed so as to incline to earth's orbit in an angle of 8 degrees; this represents the orbit of the moon, the moon itself being represented by a silver Nth. The superior
planes are Mars, Jupiter, and Saturn, in this order : Mars stands next without our earth ; next to that, Jupiter ; and outermost of all, Saturn. By the handle, the revolutions of these planets are represented. While • the earth is carried round the sun by 365 turns and a quarter of the handle, Mercury is car ried round the sun in 88 turn; and Venus in .224 ; which represents that the length of the year in Mercury is 89 of our days, and the length of the year in Venus is equal to 224 of our days. Mars performs his revolution about the sun in 687 turns of the handle, Ju piter in 4332 turns, which answers to 4332 revolutions of our earth about its own axis; and last of all, Saturn, in 10759 turns, coin. pletes his revolution, which is the length of the Saturnian year, and is equal to about 30 of our years.