[328]. This Comet was observed by John Lukens, Esq. of Philadelphia, on the 20th of January, 1784. This respectable practical astronomer communicated his discovery of it to Dr. Rittenhouse the next day, on the evening of which, (“assisted by Mr. Lukens and Mr. Prior,”) he ascertained the then apparent place of this comet. Dr. Rittenhouse’s communication to the society, on this subject, gives also the apparent place of the comet on the 17th of February, being the last time the weather permitted him to see it: the result of his intermediate observations is also stated.
[329]. These observations were made in Philadelphia, by Dr. Rittenhouse, at sundry times in the years 1784, 1785, and 1786, on the new planet, or Georgium Sidus; and on the Transit of Mercury over the Sun’s disk, on the 12th of November, 1782. The same communication also states the geocentric places of the Georgium Sidus, at several different dates between the 1st of April, 1762, and the 14th of March, 1784, both included; as observed by Mr. James Six, at the city of Canterbury in England.
[330]. This eclipse was observed by Andrew Ellicott, Esq. at the city of Washington, as follows; viz.
 | 6.h | 39′. | 1″,25 |  | Annulus completed | | ||
| April 3d. | 6. | 43. | 15,25 | Do. broken | M. Time. | |||
| 7. | 55. | 37,75 | End of Eclipse. |
A letter from the celebrated French Astronomer, Lalande, to Dr. Rittenhouse, on the same subject, has been already given.
[331]. In this letter, Dr. Rittenhouse merely informs Mr. Patterson, in general terms, of the time when he first observed this comet, its then place, and its course through several of the constellations, until the 8th of February; on the evening of which day, he saw it for the last time. It is presumed that Dr. Rittenhouse’s state of health, at that period, would not admit of his making more definite observations on this comet.
[332]. This desideratum in astronomical science had long engaged Dr. Rittenhouse’s attention; and it is confidently said by one of his intimate friends, that, in the latter part of his life, he had actually written much on the subject of Pendulums; intended, probably, for publication. But, unfortunately, the manuscript-book, which contained what he had thus written, can not now be found.
In the paper mentioned in the text, he remarks, that “the invention and construction of time-keepers may be reckoned among the most successful exertions of human genius. Pendulum-clocks especially,” says he, “have been made to measure time with astonishing accuracy; and, if there are still some causes of inequality in their motions, the united efforts of mechanism, philosophy and mathematics, will probably, in time, remove them.”
Indeed no man has done more, none perhaps so much, towards removing the imperfections in chronometers, to which he alludes, as this great mechanician himself. His admirable time-piece, now in the possession of the Philosophical Society at Philadelphia, constructed by him, on an improved plan of his own, affords ample proof of the “astonishing accuracy” (as he expresses it) to which the pendulum-chronometer may be brought. A description of the mechanism of this extremely accurate time-piece, as well as of the principles on which its superior correctness depends, is inserted in the Appendix.
[333]. The appropriate location of “The American Philosophical Society” is the city of “Philadelphia,” where their meetings must be held in conformity to their charter. Monticello, Mr. Jefferson’s residence in Virginia, is situated at the distance of about two hundred and seventy miles from the capital of Pennsylvania.