tneafuring Heights zvrtb the Barometer. 7 \ 9 
were determined, would be extremely tedious and unin- 
terefting. That fome idea may however be formed of 
the degree of accuracy with which they were afcertained, 
it will be fufficient to obferve, that the requifite angles 
were taken with an aftronomical quadrant of a foot ra- 
dius, made by Mr. sisson, and curioufly adapted for the 
meafurement of horizontal or bale angles; which, as well 
as thofe of the vertical kind, might always be determined 
thereby to within ten feconds of the truth. The bafes 
were meafured with care ; and, in order to afcertain the 
diftances, the three angles of each triangle were, as 
often as poffible, actually obferved with the quadrant. 
That the variation of the line of collimation of the in* 
ftrument, which was found to alter in carrying, might 
occalion no error, one or more of the angles of elevation, 
at each ftation, were taken on the arc of excefs, as well as 
on the quadrantal arc. I11 all cafes, the ufual <,) allow- 
ances were made for curvature and refraction : and for 
the correction of the la ft, fometimes the angles of de- 
( 1 ) If the fquare of the diflance be divided by the diameter of the earth, the 
quotient will give the curvature of the globe on that diflance, or the excefs of 
the apparent above the true level : and, by Mr. maskelyne’s rule, the fquare of 
the diflance being divided by the diameter of the earth, augmented by one- 
fourth part, we have the allowance for curvature and refraction; which laft is 
fuppofed to raife the objeCt, by an angle equal to that of a great circle fubtended 
by one-tenth part of the diflance. 
4 Z 
VOL. LX VII. 
preffion 
