The Automaton of Christian Huygens was, in all pro bability, the first planetary machine that represents the motions of the heavenly bodies agreeably to the true sys tem. It is supposed to have been contrived and construct ed during the ingenious mechanist's residence at Paris, between the years 1665 and 1681, though its description was not published till the year 1703. The world is indebt ed to this author for his beautiful method of determining such trains of wheelwm k, as shall produce, the planetary periods with great precision ; which method will be ex plained presently: but some doubt may be entertained whether he made the best use of his knowledge in the construction of his own mechanism. After he had com puted numbers for the teeth of his wheels and pinions for an exact solar year, he adopted a train that gave motion to his annual arbor, or prime mover, in 365 clays, instead of 365.242, &c. thereby vitiating all his original compu tations; and Benjamin Martin informs us, that this ma chine was the original from which the orreries of his time derived their chief constituent parts. Many of the larger wheels of the automaton were composed of rings placed a little eccentrically, and kept in their places by rollers, so that an alternate increase and decrease of velocity was effected by a variation in the lengths of the radii, by means of long pinions lying across the cont•ate edges of the in dented rings; while some of the last wheels of certain trains had their teeth cut into unequal sizes, so as to pro duce the same effect by that inequality. In this machine, all the trains were kept in constant natural motion, by a clock regulated by a balance and balance spring, before the pendulum had yet been applied as the regulating agent. The places of the aphelia and ascending nodes of the pla nets, as given by Huygens, were taken from the tables or.
Ricciolus, for the 1st Jan. 1682, which, therefore, was probably the time when the machine was finished, and ready for rectification.
Roemer was mathematician to the French King, (Louis XIV.) at the time when Huygens resided at Paris, and probably caught from him a fondness for astronomical mechanism ; for we find him presenting Flamsteed with a satellite instrument of his contrivance in the year 1679. It is not improbable but that Huygens may have been con sulted by Roemer on the eligibility of his plan, which, being calculated to produce the motions of Jupiter's sa tellites only. was sooner made, and did not interfere with Huygens' calculations. The construction, too, was dif ferent; for Roemer introduced a set of concentric tubes, revolving within one another, which received, on their lower extremities, each the second wheel of as many pairs, while the first wheel of each pair was made fast to one common arbor, from which the motion commenced, on a supposition that its revolution was performed in an exact week : hence the periods of the four satellites were produced by four pairs of wheels in as many correspond ing revolutions of the arms carrying the satellites round their primary, when attached by friction to the upper ends of their respective tubes. The method of determining the values of these periods will be explained hereafter. As Huygens' automaton was the prototype of the orreries that followed it, so Roemer's satellite instrument dictated the construction, subsequently adopted, for the common planetarium, where a system of revolving concentric tubes constitutes the basis.
In 1735, Roemer published his "Basis ?lstronomice," in which he gives a description of a planetarium con structed under his direction, which was finished in 1697, and which appears to have been the very model from which the ordinary planetarium has had not only its form of con struction, but its numbers copied. Like the satellite in
strument, it has the system of concentric tubes, carrying each a single wheel below, and an arm or radius vector above, in periods produced by as many pairs of wheels, that constitute so many fractions of a year, which is the period fixed on for the revolution of the common arbor, that has all the first moving wheels affixed to it. This is certainly a much more simple construction than that of the automaton, but then it admits of no other than mean or equable motions, and constant distances.
About the beginning of the eighteenth century, spheres began to be constructed in France, containing planetary motions, and put in motion by clockwork. Martinot's armillary sphere was made, with the sun in the earth's place, in the year 1701 ; while Pigeon's sphere had the sun in the centre, which is said to have been constructed about the same time ; hence, it should seem, the struggle between the two s) stems of Ptolemy and Copernicus had not been filially relinquished at the commencement of the eighteenth century. In Martinot's sphere, the computa tions made the year only 365 days, but in Pigeon's the year consisted of 49' ; so that the latter must have been preferable in more respects than one.
About the year 1715, when Rowley and Graham distin o-uished themselves in London, by their mechanical skill, the former was applied to by the Earl of Orrery, to make a grand machine for representing the motions of the pla nets in their true periods and orbits, as nearly as could be effected by mechanical means; and when the complex machinery was finished, with some deviations limn the construction of the automaton of Huygens, the maker called it, in honour of his noble employer, an ORRERY, which appellation has continued to designate those more complex machines, in which both the revolutions round the sun, and also some of the rotations round the axes, have been mechanically effected at the same time; where as those which exhibit only the periodic revolutions, or motions in circular concentric orbits, have been called Planetaria, according to the name originally selected by Roemer. By the accounts that have been transmitted to us, some of the contrivances in Rowley's grand orrery, of which there is a specimen preserved at the Royal Obser vatory at Kew Gardens, were introduced by Graham ; but the weight of metal and complexity of the parts, chiefly concealed from the sight, will probably prevent so expensive a machine from being constructed in future; and particularly as five additional planets have been disco vered since its formation ; and as the general principles of calculation and construction of planetary mechanism are better understood in the nineteenth, than they were in the eighteenth century. Some of the orreries that have been recently made, are calculated not to astonish, but to in struct the student in astronomical science. And, for this reason, we propose not to waste our pages in the minute description of inventions that no longer interest the rea der, but to describe those machines only, which, from their improved constructions, are best calculated to con vey, at the same time, both amusement and instruction.