VOL. LVIII.3 PHILOSOPHICAL TRANSACTIONS. 567 



The tremor of the air, caused by the sun's rays, was often very great ; and to 

 avoid any error that might arise from the fluttering of the marks, we intermitted 

 our operations sometimes for 5 or 6 hours in a day, and were often obliged to 

 make use of the morning or evening twilight. In the continuation of the line, 

 a person was left at the mark, behind the instrument, till another mark was set 

 forward, to prove with a plummet that its centre was not moved. The visto cut 

 through the woods, in this work, was about 8 or Q yards wide, and in general 

 seen about 1 miles, beautifully terminating to the eye in a point. The zenith 

 distances of the stars, for determining the celestial arc, answering to the inter- 

 val of the parallels of the northernmost and southernmost points of the lines, 

 were made with an excellent sector of 6 feet radius, constructed by Mr. John 

 Bird. 



In the course of the work, for dividing the provinces of Maryland and Penn- 

 sylvania, the following lines were traced out, that offered themselves for deter- 

 mining the length of a degree of latitude.. 



In fig. 3, pi. l6, let N represent the northernmost point, and a the most 

 southern of the said lines. Beginning at n, a meridian was traced from n to 

 p= J 4 mil. 64 cha. 8 links. In this line there were some hills, which were 

 measured horizontally with a level, but the plains were measured with a chain. 

 PC = 2m. TQch. 27 lin. ; c being in the parallel of latitude with p, which was 

 determined by the sector, dc a meridian =: 5m. 2ch. 43lin. in which are 3 or 

 4 small ascents and descents. The points b, d, e, and m, are in a right line. 

 BD = 22cha. 5llin. and the angle cdm = 86" 32^-' nearly. Hence, b is south 

 of D = Ich. 361in. = D ^. The line ab = 81m. 78ch. 3llin., in which is one 

 gentle rising hill, about half a mile over ; all the rest of the line is an entire 

 level or plain. 



These measurements, expressed in English statute miles and parts of the same, 

 were made with a chain, established from a brass statute yard, which was proved 

 and corrected, in the course of the work, by another statute chain, kept only 

 for that purpose, made from the said brass yard. They were only designed for 

 dividing the provinces of Maryland and Pennsylvania : the same' lines were re- 

 measured afterwards with wooden rectangular levels, for the purpose of deter- 

 mining the length of a degree of latitude, as will appear in the sequel of this 

 work. 



The point c was placed in the parallel of latitude of p, thus. Let n, fig. 4, 

 represent the north pole of the terrestrial globe ; p and r two places lying in the 

 same parallel of latitude rcp ; pr an arch of a great circle = lO' joining the 

 said points ; and pn, rn two meridians, pn, or the complement of the latitude 

 of p, being = 50° l6' 42'', the angle npr, or the azimuth of the great circle 

 PB, was found by calculation to be 89° 55' 51*. The going of the clock being 



