566 PHILOSOPHICAL TKANSACTIONS. [aNNO I768. 



taken notice of before, the learned Father Boscovich has shown, may produce a 

 very considerable deviation of the plumb-line, in the elaborate treatise of the 

 measure of a degree of the meridian between Rome and Rimini, taken by him- 

 self and his learned coadjutor. Father le Maire. 



XLII. Observations for Determining the Length of a Degree of Latitude in the 

 Provinces of Maryland and Pennsylvania, in North America. By Messrs. 

 Mason and Dixon, p. 274. 



In this work, the first thing to be considered was, how to continue a right 

 line : and this was done by setting up marks with the assistance of an equal 

 altitude or transit instrument (for it was contrived so as to serve either purpose 

 at pleasure), made by Mr. John Bird, of the same construction with that de- 

 scribed by M. Le Monnier, in the preface to the single volume of the French 

 Histoire Celeste. The cylindrical ends of the cross axis of the telescope were 

 laid in two angles of the supporters, which rose perpendicularly from a horizontal 

 bar, that was fastened firmly to the upper part of the vertical axis. The axis of 

 the telescope was set truly horizontal, by a spirit level hung on its cylindrical 

 ends. The brass frame, which receives the vertical axis, was screwed to a post 

 fixed in the ground, in the direction of the line which was to be continued. 

 When the vertical wire in the telescope was brought to bisect any mark, it was 

 kept in that direction, by confining firmly, between two pushing screws, a hori- 

 zontal arm that projected from a collar that surrounded the vertical axis ; and, to 

 prove that a small shock would not alter its position, a small pressure was 

 applied against one of the supporters, which being removed, it was carefully 

 noted, whether the wire returned again to bisect the mark. 



At every station, or mark, the telescope was turned 2 or 3 times after the mark 

 was fixed in the line, to prove that the said mark was truly set. In general, the 

 distances between the marks did not exceed a mile, nor were they less than half 

 a one. The telescope magnified about 25 times. Three or 4 marks were always 

 left standing, and on a little rising ground they could all be seen in a right line, 

 the vertical wire in the telescope bisecting their centres without sensible error. 

 The marks used in continuing the lines, were concentrical circles of black and 

 white, painted on both sides of a board 14 inches square. This board moved in 

 mortices made in two posts, which were driven into the ground ; and when 

 the centre of the mark was brought, by means of signals, into the line, it was 

 fastened by wedges to the posts. By means of a plummet, a peg was driven into 

 the ground, and a notch cut in it, under the centre of the mark, to secure the 

 line. In the evening, when we left ofl^, a mark was placed before, and 2 or 3 left 

 behind us ; and in the morning the instrument was again set up in the same 

 place, to prove that the marks were not moved. 



