ORRERY

ORRERY, a curious machine for representing the motions or phases of the heavenly bodies. See ASTRONOMY, n^o 317.

It would be too great an undertaking here to give an account of the mechanism of the larger sort of orreries, which represent the movements of all the heavenly bodies; nor indeed can it be done either by diagram or description, to render it intelligible to the most discerning reader: but, instead of that, we shall exhibit an idea of the theory and structure of an useful, concise, and portable planetarium, which any gentleman may have made for a small expence, and will exhibit, very justly, the motions of all the primary planets about the sun, by wheel-work; and those that have secondaries, or moons, may have them placed about their primaries moveable by the hand, so that the whole shall be a just representation of the solar system, or true state of the heavens, for any given time of the year.

In order to this we must compare, and find out the proportion, which the periodical times, or revolutions of the primary planets, bear to that of the earth: And they are such as are expressed in the table below, where the first column is the time of the earth's period in days and decimal parts; the second, that of the planets; the third and fourth are numbers in the same proportion to each other: as,

If we now suppose a spindle or arbour with six wheels fixed upon it in an horizontal position, having the number of teeth in each, corresponding to the numbers in the third column, viz. the wheel AM of 83 teeth, BL of 52, CK of 50 (for the earth), DI of 40, EH of 7, and FG of 5; and another set of wheels moving freely about an arbor, having the number of teeth in the fourth column, viz. AN of 20, BO of 32, CP of 50 (for the earth), DQ of 75, ER of 83, and FS of 148; then, if those two arbors of fixed and moveable wheels are made of the size, and fixed at the distance from each other, as here represented in the scheme, the teeth of the former will take those of the latter, and turn them very freely when the machine is in motion.

These arbors, with their wheel, are to be placed in a box, of an adequate size, in a perpendicular position: the arbor of fixed wheels to move in pivots at the top and bottom of the box; and the arbor of moveable wheels to go thro' the top of the box, to a proper height, on the top of which is to be placed a round ball, gilt with gold to represent the sun. On each of the moveable wheels is to be fixed a socket, or tube, ascending above the top of the box, and having on the top a wire fixed, and bent at a proper distance into a right angle upwards, bearing on the top a small round ball, representing its proper planets.

If then on the lower part of the arbor of fixed wheels be placed a pinion of screw-teeth, a winch turning a spindle with an endless screw, playing in the teeth of the arbour, will turn it with all its wheels; and these wheels will move the others about, with their planets, in their proper and respective periods of time, very exactly. For while the fixed wheel CK moves its equal CP once round, the wheel AM will move AN a little more than four times round, and so will nicely exhibit the motion of Mercury; and the wheel

FG will turn the wheel FS about $\backslash frac\{1\}\{29,5\}$ round, and so will truly represent the motion of Saturn: and the same is to be observed of all the rest.