VOL. LVIII.] PHILOSOPHICAL TRANSACTIONS. 505 



and the like observations repeated there, on the same stars, which had already 

 been observed at the northernmost point. 



This plan of a measure of a degree in North America, Messrs. Mason and 

 Dixon submitted to the consideration of the council of the r, s., and offered to 

 carry it into execution, at the expence of the society, if they thought proper. 

 The council determined that so useful and important a work should be com- 

 pleted, and accordingly sent out instructions to Messrs. Mason and Dixon for 

 the regulation of their operations ; particularly requiring them to measure the 

 lines carefully over again with fir-rods, which they sent to them, together with 

 a brass standard, of 5 feet, with which the rods were to be compared frequently, 

 and the difference noted, and also the height of the thermometer at the time ; 

 (or the lines had been all measured before with a standard chain, which, though 

 sufficient for the common purposes of surveying, was by no means to be de- 

 pended on in so nice an operation as that of measuring a degree of latitude. 

 Mr. Penn was pleased, at the request of the r. s., to grant the further use of 

 his sector, before mentioned, and other instruments, to the observers, for com- 

 pleating this measure. 



The method pursued in this work, is that which the level disposition of the 

 coimtry pointed out. But the result may be expected to be more accurate on 

 this account, as measures taken in a straight line, and on a level surface, are 

 known to be capable of great exactness ; and no adventitious errors are here in- 

 troduced from any possible errors of a chain of triangles. Messrs. Mason and 

 Dixon having also determined the angle which the oblique line made with the 

 meridian, by proper astronomical observations, and the amplitude of the arch of 

 their meridian line by several observations of zenith distances of fixed stars, 

 made at both ends of the meridian, with the limb of the sector turned both east 

 and west at each extremity ; this measure of a degree seems to be as well stated, 

 and as much to be depended on, as any that has been made ; and will probably 

 be thought a valuable addition to the other measures of degrees, which have 

 been taken with great care and pains, by various learned men, particularly the 

 members of the Royal Academy of Sciences at Paris, who have acquired so 

 mu<!h just reputation by their valuable labours bestowed on this subject. 



It may not be improper to remark, that the level disposition of the country 

 this degree passes through, which, it appears, also obtains further to the south, 

 and in a great measure to the north of the limits of the same, gives some 

 advantage to this measure, with respect to the use that may be made of it 

 in inquiring into the figure of the earth ; as there is no room for suspicion that 

 the plumb-line of the sector could be deflected materially from its proper posi- 

 tion by the attraction of any mountain, or even elevated ground of a more 

 moderate height, continued for a gpeat length ; which latter circumstance, not 



