It is about fifty years since this prince built an observatory near the gardens of Schwetzingen, two leagues from the city of Manheim; and there Father Christian Mayer pursued his astronomical labours for several years. But about the year 1772, the same prince erected another and a more magnificent edifice (of 108 Rhenish feet in height,) at Manheim, for the same purpose: where Father Mayer made numerous and highly estimable observations; as may be collected from his work, entitled, _De novis in Cælo Siderio Phænomenis_. Lalande places this observatory in N. Lat. 49° 29′ 15″. ----- On a perusal of the answer to Mr. Mayer’s communication (in the Appendix,) it will be found, that the “eminent utility,” which he expected to result, at some future day, to astronomical science, from a prosecution of such discoveries as he had recently made among the fixed stars, had been long before anticipated by our Astronomer. In that answer Mr. Rittenhouse mentions, that he is induced to request his correspondent’s acceptance of a copy of the Oration he had delivered before the American Philosophical Society, “some years” before:—“because,” says the writer, “I therein gave my opinion, that the fixed stars afforded the most spacious field for the industry of future astronomers; and expressed my hopes, that the noblest mysteries would sometime be unfolded, in those immensely distant regions.” This early opinion of his own concerning the fixed stars, to which Mr. Rittenhouse refers in his letter, is expressed in his Oration, in this short paragraph: “If astronomy shall again break those limits that now confine it, and expatiate freely in the superior celestial fields,—what amazing discoveries may yet be made among the fixed stars! That grand phænomenon the Milky way, seems to be the clue, that will one day guide us.” Such were the expectations entertained by our Philosopher, more than three years before the date of Mr. Mayer’s communication of his discovery to the Philosophical Society;—a discovery which Mr. Rittenhouse, in his letter to that great astronomer, styles “excellent;” and one that proves his own “presage” to have been well founded. He, at the same time, modestly suggests to Mr. Mayer, the institution of a comparison between the many observations he had already made, in order to determine, whether the several changes observed will agree with any imagined motion of our system; remarking, that those he had communicated, seemed to favour such a supposition.