Mr. Maskelyne, the celebrated English Astronomer before mentioned,[127]—in a letter to Dr. Smith, of the 26th of December, 1769—expressed a wish “that the difference of Meridians of Norriton and Philadelphia, could be determined by some measures and bearings, within one-fiftieth or one-hundredth part of the whole; in order to connect,” continues Mr. Maskelyne, “your observations of the Longitude of _Norriton_ with those made by Messrs. Mason and Dixon, in the course of measuring the degree of Latitude.”—This request of the Astronomer Royal was communicated to the Philosophical Society of Philadelphia; in consequence of which, Dr. Smith, Mr. Lukens and Mr. Rittenhouse, were appointed to make the terrestrial measurement required. These gentlemen, having taken to their assistance Mr. Archibald M’Clean and Mr. Jesse Lukens, two able and experienced Surveyors, commenced their operations at Norriton, early on the 2d day of July following, and completed their survey on the 4th day of that month. The Report of the able Committee, to which this business was assigned by the American Philosophical Society, is also contained in the first volume of the Transactions of that learned Body. After giving various calculations, resulting from the operations of that committee, the Reporter says—“Hence, by the above measurement and work, we get Norriton Observatory 52″ of time West of the Observatory in the State-house Square; which is exactly what we got by that excellent element, the external contact of Mercury with the Sun, Nov. 9th 1769.”—“The external contact,” continues the Reporter, “gave it something more; owing, no doubt, to the difference that will arise among Observers, in determining the exact moment when the thread of light is compleated: and the mean of all our Observations gives the difference of Meridians, between Norriton and Philadelphia, only 4″ of time more than the terrestrial measurement, and the external contact of Mercury, gave it,—which may be taken as a very great degree of exactness; if we consider that the difference of Meridians, between the long-established Observatories of Greenwich and Paris, (as Mr. De la Lande writes, Nov. 18th 1762,) was not then determined within 20″ of time—For, he says, ‘some called it 9′ 15″; others, 9′ 40″;’ but that he himself commonly used 9′ 20″, though he could not tell from what Observations it was deduced.[128]—Finally, the Report fixes Philadelphia to be 5^h 0′ 37″, and Norriton, 5^h 1′ 29″ West from Greenwich.[129] The Latitude of Norriton, as deduced from the actual mensuration just mentioned, connected with Observations previously made by Mr. Rittenhouse—predicated also, in part, on antecedent Calculations of Messrs. Mason and Dixon, who, (having been furnished with a complete Astronomical Sector,) had ascertained the southernmost point of the City of Philadelphia to be in Latitude 39° 56′ 29″,4. N.—is stated, in the same Report, as being 40° 9′ 31″. It came out, by