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70 The year 1769 is memorable in the annals of astronomy. During that year occurred the transit of Venus — a phenomenon which offers the best means for calculating the distances between the heavenly bodies. It had up to that time never been satisfactorily observed. No man then living could ever have the opportunity again because it would not recur for one hundred and five years Astronomers all over the world were alive to its importance. Arrangements were made for taking such observations as were possible in the capitals of Europe, and the governments of England and France sent expeditions for the purpose to Otaheite, Hudson's Bay, and California. As early as June 21st in the preceding year, Rittenhouse read before the American Philosophical Society a series of calculations showing the time and duration of the coming transit. The Legislature of Pennsylvania gave £200 sterling toward the expense of buying a telescope and micrometer and the other outlays, and on the 7th of January, 1769, the society appointed three committees to make observations in three different localities. One of these committees consisting of Rittenhouse, Dr. William Smith, John Lukens, and John Sellers, was to repair to the home of Rittenhouse at Norriton, and to him were intrusted all of the preliminary arrangements. In November he began the erection of an observatory, which was completed in April. He continued for months a series of observations to determine the exact latitude and longitude of the place, and to test the accuracy of his time-pieces. Thomas Penn sent from Europe a reflector, used by Smith; a set of glasses intended for Harvard University, but which came too late to be forwarded, Rittenhouse fitted into a refractor for Lukens; and his own telescope he retained. Several other necessary instruments, including a device for keeping time, he made with his own hands, and, like