The “Description” here referred to, in Mr. Rittenhouse’s own hand-writing, is new before the writer of these Memoirs; and is thus endorsed by the Rev. Mr. Barton—“Original Description of Mr. Rittenhouse’s Orrery, first communicated to Thomas Barton.”—For the satisfaction of those, who may not have an opportunity of seeing the American Philosophical Society’s Transactions, in which this short account of the Orrery was afterwards published; and, as this _original_ description of it differs somewhat from the _printed_ one, it is presumed that the introduction of the former into this work, will not be unacceptable to the reader. The impossibility of conveying to the mind of any one, even the most intelligent and skilful, by means of either any delineation upon paper in the nature of a diagram, or by words, an adequate idea of so complex and multiform a machine as the one now about to be described, will instantly be conceived. Indeed no description, alone, can render the nature of its construction, and the many curious and useful purposes it is capable of answering, perfectly intelligible to the most scientific Astronomer. Mr. Rittenhouse’s very concise description of his Orrery will, therefore, necessarily be found defective: it is thus worded by himself. --- “DESCRIPTION OF A NEW ORRERY. “This Machine is intended to have three faces, standing perpendicular to the horizon: that in the front to be four feet square, made of sheet-brass, curiously polished, silvered, and painted in proper places, and otherwise ornamented. From the centre arises an axis, to support a gilded brass ball, intended to represent the Sun. Round this ball move others, made of brass or ivory, to represent the Planets: They are to move in elliptical orbits, having the central ball in one focus; and their motions to be sometimes swifter, and sometimes slower, as nearly according to the true law of an equable description of areas as is possible, without too great a complication of wheel-work. The orbit of each Planet is likewise to be properly inclined to those of the others; and their Aphelia and Nodes justly placed; and their velocities so accurately adjusted, as not to differ sensibly from the tables of Astronomy in some thousands of years. “For the greater beauty of the instrument, the balls representing the planets are to be of a considerable bigness; but so contrived, that they may be taken off at pleasure, and others, much smaller, and fitter for some purposes, put in their places. “When the Machine is put in motion, by the turning of a winch, there are three indexes which point out the hour of the day, the day of the month, and the year (according to the Julian account,) answering to that situation of the heavenly bodies which is then represented; and so continually, for a period of 5000 years, either forward or backward.