the mean Dmjify of the Earth . 755 
According to the foregoing method the plan of the 
ground was divided into 20 rings by equidiftant con- 
centric circles, defcribed about each obfervatory as a cen- 
ter; and each quadrant was divided into 12 parts or 
fedtors by lines forming, with the meridian, angles whofe 
lines are in arithmetical progrellion ; by which means 
the fpace in each quadrant was divided into 240 fmali 
parts, making almoft a thoufand of fuch parts in the 
whole round foreach obfervatory, or near 2000 for the 
two obfervatories. This was judged to be a fufficiently 
great number of parts to afford a very confiderable de- 
gree of accuracy ; or at leaft that number was as great, 
and the parts as fmali, as was well confiftent with the 
degree of accuracy afforded by the number of points, 
whofe relative altitudes had been determined. 
In this divilion the common breadth of the rings, or 
the common difference of the radii, is 666 - feet; arid 
the common difference of the fines of the angles formed 
by the radii and the meridian is ~th of the radius ; and 
confequently, thofe angles are expreffed in degrees and- 
minutes as here follows, viz. 4 0 47', 9 0 36', 14 0 sqV 
19 0 28', 24° 37 / , 30° o', 35 0 41', 41 0 487, 48° 35V 
56° 26^, 66° 26'-, 90° o'. 
Tab. ix. contains a fmali plan of the principal and 
moft centra] part of the ground, accurately divided in the 
5 A 2 above 
