The establishment of mechanical and physical science on an experimental basis, in the sixteenth and seventeenth centuries, is an event, or rather a series of events, for which the world cannot be sufficiently grateful. The splendid discoveries which were from time to time announced, and the various machines constructed for their illustration, directed men's minds strongly to mechanical pursuits. The results were of great value to civilization; and even when the objects sought for were frivolous, they were not altogether useless, as in the construction of clocks which, in addition to their ordinary duties, were made to undertake many wonderful and unnecessary things; automata of various kinds, which accomplished objects equally wonderful and unnecessary; and planetary machines for representing the motions of the heavenly bodies. The value of these things consisted in gratifying the public taste for ingenious toys, in fostering mechanical skill, and in supplying clever combinations, which have subsequently been reproduced in really useful automatic machines.

The term orrery is sometimes applied to planetary machines, and the origin of this word is stated by Desaguliers to have arisen from the following circumstances.—About the year 1700 Graham contrived a movement for exhibiting the motion of the earth about the sun, together with the moon revolving about the earth. This machine was sent to an instrument-maker's, for the purpose of being packed up with some other instruments about to be forwarded to Prince Eugene; but the maker having copied it, sold his copy to the Earl of Orrery, who it appears showed it to Steele, and he, referring to it in one of his lucubrations, called it an orrery,—a name which was at once adopted, and has since been retained.

Graham's machine, however, was by no means the first of its kind. The Chinese are said to have constructed planetary machines 2000 years before the Christian era. Archimedes and Posidonius also constructed them; and Cicero makes use of both machines in his argument on design in creation. (De Nat. Deor., lib. ii., cap. 34 and 35.) During many centuries, planetary machines consisted of moveable spheres, with the earth in the centre,—a construction which continued in use until about fifteen years after the death of Copernicus, when the last celebrated example of a machine illustrative of that system was placed in the library of the Pantheon at Paris. Huyghens and Römer also constructed machines for illustrating the Copernican system. Huyghens first introduced a systematic method of calculating the necessary wheel-work for the machine which he named the automation. This long continued to be a pattern in the construction of orreries; as was also Römer's instrument, intended chiefly to exhibit the orbital motions of the planets. Römer also invented a machine for representing the motions of Jupiter's satellites about their primary, together with the motions of the latter about the sun. In 1679 he presented a machine of this kind to the English astronomer Flamsteed. Among other great names which have been concerned in the construction of planetary machines, must be mentioned those of Dr Thomas Young and the Rev. W. Pearson, who furnished the plan for the planetarium of the Royal Institution of London.

Planetary machines have been variously named according to the objects intended to be attained by them. Thus we have the planetarium, which represents the orbital motions of the planets about the sun; the tellurium and the luna-

rium, for exhibiting the motion of the moon about the earth, and that of the earth about the sun, together with the principal phenomena which accompany the changes in their relative positions,—such as the succession of day and night, and the variable length of both according to the season of the year; the eclipses of the sun and moon; variations in the moon's latitude, velocity, and distance from the earth; the progressive motion of her apogee, and the retrogradation of her nodes, &c. The satellite machine by Römer, already referred to, is the last of the four planetary machines to which distinct names have been given corresponding to the phenomena they are intended to exhibit; but they are all generally included under the term orrery.

The most important part of an orrery is the mechanism of the planetarium, for exhibiting the paths of the planets about the sun, and their relative periodic times, together with the motions of the satellites about their primaries. The former object is usually attained by means of a system of upright concentric tubes of different lengths, the innermost being the longest; to the upper extremity of each is attached a radius vector, which revolves once during each revolution of the tube; the lower ends of the tubes form the arbors of as many toothed wheels, which are driven either by pinions adjusted to a vertical axle called the annual arbor, or they are moved by those pinions through the intervention of a train of wheels. The calculation of the relative number of teeth to be given to the wheels and pinions, in order to produce the required motions, belongs to that part of mechanics which treats of wheel-work.

It is not necessary to dwell at greater length on the construction of a machine which is at best a clumsy, useless, and expensive toy. As science advances, the instruments of research become more and more perfect; and although, as the great German chemist expresses it, it is not the instrument that does the work, but the mind of the inventor (Das Instrument macht ja das Werk nicht, sondern der menschliche Geist), still the instruments of the day will fairly represent the science of the day, and in this science the planetary machine has no place. That this machine never did, and never can, represent the truth as it is in nature, will be evident from the following computation, which we borrow from a great living astronomer—

"Choose any well levelled field or bowling-green. On it place a globe two feet in diameter; this will represent the sun: Mercury will be represented by a grain of mustard-seed, on the circumference of a circle 164 feet in diameter for its orbit; Venus, a pea, on a circle 284 feet in diameter; the earth also a pea, on a circle of 450 feet; Mars, a rather large pin's head, on a circle of 654 feet; Juno, Ceres, Vesta, and Pallas, grains of sand, in orbits of from 1000 to 1200 feet; Jupiter, a moderate-sized orange, in a circle nearly half a mile across; Saturn, a small orange, on a circle of four-fifths of a mile; and Uranus, a full-sized cherry or small plum, upon the circumference of a circle more than a mile and a half in diameter. As to getting correct notions on this subject by drawing circles on paper, or, still worse, from those very childish toys called orreries, it is out of the question." (Herschel's Astronomy, 1848.)