578 



PHILOSOPHICAL TRANSACTIONS. 



[anno 1768. 



with respect to this particular measure of a degree, at the end of his Introduction to Messrs. 

 Mason and Dixon's account of the same, by arguing, perhaps too far, from the level disposition of the 

 country through which the degree passes. But Mr. Henry Cavendish has since considered this 

 matter more minutely ; and having mathematically investigated several rules for finding the attraction 

 of the inequalities of the earth, has, upon probable suppositions of the distance and height of the 

 Allegany mountains from the degree measured, and the depth and declivity of the Atlantic ocean, 

 computed what alteration might be produced in tlie length of the degree, from the attraction of the 

 said hills, and the defect of attraction of the Atlantic ; and finds the degree may have been dimi- 

 nished by €0 or 1 00 toises from these causes. He has also found, by similar calculations, that the 

 degrees measured in Italy, and at the Cape of Good Hope, may be very sensibly affected by the 

 attraction of hills, and defect of the attraction of the Mediterranean Sea and Indian Ocean. 



The rules which Mr. M. used in calculating the ratio of the equatorial diameter to the polar axis, 

 from the North American degree, compared with those measured in Peru and Lapland, are those 

 given by Mr. John Robertson, Librarian to the r. s., in his elements of Navigation, p. 597, as 

 deduced by him from Dr. Letherland's Geometrical Analysis of the problem, which he has also given 

 to the public in the same place, together with some other problems depending on it, which were 

 necessary to complete the subject. 



XLIII. Astronomical Observations, made in the Forks of the River Brandiwine 

 in Pennsylvania, for determining the going of a Clock sent thither by the Royal 

 Society, in order to find the Difference of Gravity between the Royal Observa- 

 tory at Greenwich, and the Place where the Clock was set up in Pennsylvania ; 

 to which are added, an Observation of the End of an Eclipse of the Moon, and 

 some Immersions of Jupiter s First Satellite observed at the same Place in Penn- 

 sylvania. By Charles Mason and Jeremiah Dixon, p. 329- 



The place where these observations were made is the northernmost point of 

 the lines that were measured for a degree of latitude, or point n, (see pi. 16, 

 fig. 3) relative to that measure; it lies 31 miles west, by measurement; and 

 10''.5 south of the southernmost point of the city of Philadelphia, as found by 

 the sector. 



Having observed the time by the equal altitudes of several fixed stars, it is 

 then added — From these observations we have the time of Capella's passing the 

 meridian, and the rate of the clock's going as follows: 



1767. 

 March. 



b 17 



D eclipsed. 

 Time per watch. 



8" 4"" 10* Eclipse of the J ended. 



8 58 46 10* 27"" 30-j Equal al- 



9 1 16 2y 41 >titude of 

 4 5 32 9 J regulus. 



The watch went very regular sider. time. 



—Hence the eclipse ended at 8" 21" 59" appa- 

 rent time, in the forks of the river Brandiwine. 



