The Description of a Complete Planetarium for Mean tions, The common planetarium is not only imperfect with respect to the numbers that constitute the wheel-work, but defective as it regards five out of the eleven primary planets ; so that we should not perform our duty to the public, if we did not lay before them the model of a ma chine that will represent the various motions in their periodic times, and in a much more perfect manner. The model that we have to offer was recently contrived by a mechanist, who has been much accustomed to exercise his talents both in the computation and construction of planetary machines, and we anticipate, that the boon we here offer to the instrument-makers will be gratefully re ceived. The greatest portion of the planetary numbers are derived from the fractions of a solar year, which we have already presented to our readers ; and the diameters of the respective pairs of wheels employed for giving the periods of the different planets, are put into proper pro portions, as well as practicable sizes; so that any mechanic, who has access to a good cutting-engine, will have no difficulty in preparing the mechanism. The construction also is so simple, that any workman, who has seen a com mon planetarium, may put all the parts together. Each planet has only one pair of wheels, forming the fraction of a solar year, except Herschel, which, from its slow motion, is incapable of being actuated by the same means, and therefore its revolution is derived from Saturn's, but in a way that is very simple, and will be easily understood and practised. In order to render the tropical mean periods as con ect as can be done by simple fractions of a moderate denomination, the sum of the teeth of each pair of wheels has been assumed on an average at about 170, so that the common distance from their respective centres may be 2.7 inches. and the number of teeth in an inch about 10, which are sufficiently strong, and not liable to unnecessary shake, when the teeth and spaces are made equal, and laid at a proper depth for action. We have compressed all the necessary information, for the use of the maker, into the form of a table, which affords hint, at the same time, the means of ascertaining the order in which the different pairs of wheels follow one another in their respective posi tions, as well as the requisite dimensions to be attended to for each separate wheel, and also the cutters that will be proper to be used when the engine is employed. When we speak of the diameter of any wheel, it must be under stood to mean the geometrical diameter, or diameter measured from what is practically called the pitch-line, ex elusive of the ends of the teeth that take hold of one another ; allowance must therefore be made for the addi tional acting parts of the teeth in which will be more or less, according to the coarseness or fineness of the teeth, as every workman accustomed to manufacture and nut together whed-work cannot but comprehend.
The in -rblnism of this planetarium is contained in an oblong franc of brass, having the upper plate connected with the ic ,vcr one by four pillars at the respective cor ners, which ;ire made fast with screws in the usual way ; each of - plates of this frame is eight inches long, and four wide ; and the length of the pillars may be from three to four inches, according to the thickness of the wheels and the distances between them. The long annual arbor carries round with it nine fixed wheels, or pinions, for ten of the planets ; Ceres and Pallas having both the same wheel in c ommon, and Herschel's not being included ; this arbor is pivoted into the two plates of the frame, and its upper pivot is prolonged, so as to receive the annual index above the circular, or rather cylindrical box, in which the frame is included when fixed on a tripod of brass. A long stem of tempered steel, turned on a lathe
perfectly cylindrical, is fixed to the lower plate in a ver tical position, round which the system of concentric brass tubes revolve within one another, so nicely fitted, that while there is but little friction there is as little play as possible. These tubes are of different lengths, varying about a quarter of an inch from each other, the innermost being the longest, and the outermost the shortest ; but all of them long enough to ascend through the cover of the cylindrical box, in which the frame is screwed fast, that each tube at its superior end may receive its respective planetary arm, or radius vector. As the wheels and pinions fixed last to the annual arbor are the drzyrry in each pair, the driven wheels corresponding thereto have each its se parate tube, which therefore revolves in its proper period, and carries along with it its planetary arm exactly as in the orrery hereafter described. The tube which carries Herchel's arm, as we have before said, derives its motion from Saturn's, and is thus effected : A wheel of 61 teeth is screwed fast to the under face of Saturn's large wheel of 206, and thus moves in Saturn's period ; then two wheels, of 112 and 60 teeth, are fixed on a short piece of small tube, and placed on the annual arbor loosely as on a stud, and while the 112 is connected with the 61, and is made to revolve thereby, the 60, moving along with the 11:2, drives the large planetary wheel 93 of Herschel, which therefore is made fast to the lower end of his tube, 61 60 and revolves in of 10748.566 days, (Saturn's pe 112 93 riod by the wheelwork ;) and the period thus effected is very near the truth, viz. 30589 333 days ; the revolution also is performed in the proper direction from west to east, like all the other revolutions. There is moreover a bridge of brass, with a tube fixed at right angles on its centre, which surrounds the two tubes of Venus and Mer cury, so as not to impede their motion, and which forms a fixed stud, round which the earth's tube and all the other surrounding tubes revolve., The use of this fixed tube is twofold : first, it separates the two innermost tubes, which are slender, from the more weighty tubes without, which it supports, while it lessens the friction ; and secondly, it supplies the means of giving motion to the moon, and of holding a graduated dial lor the indica tion of the sun's place, while the annual bar carries the index at its remote end to point to it ; the dial itself being indented into 235 teeth, and driving a pinion of 19 on the annual bar round the earth's stem in a lunation. The moon's stem, fixed to the small revolving tube connected with the pinion of 19, becomes moreover the index to a small plate of 291 divisions, carried by the earth's stem, and points out, in a distinct manner, the day of the moon's age, while the annual hand points out as clearly the day of the month engraven on a circle on the cover of the machine. Indeed, if a quadruple spiral were drawn in stead of a circle, the leap year would also be indicated, as the annual train gives a period not varying, more than a single second, if so much, from the true solar year.