1ET. XVI.

OF THE EARTH.

26 l

principal measure, I shall pass over the peculiarmethods and operations they employed, and pro-ceed to those of the moderns, which are farmore accurate and scientific. Riccioli attemptedto measure the earth according to a methodmentioned by Kepler. It was known from ob -servation, that heavy bodies, in falling, tendtowards the center of the earth. And as thedistance of any two places upon the surface ofthe earth may be considered as the base of a tri-angle, whose vertex is at the center, he mea-sured a large base of this kind, in the most ac-curate manner possible, and found the angleswhich it made with a plumb line at each of itsextremities. The sum of these angles, by a pro-perty in geometry, being taken from a hundredand eighty degrees, gave him the angle at thevertex ; and as he had now obtained the mea-sure of an angle at the center of the earth, andthe length of a corresponding arc upon its sur-face, it was easy, by the rule of proportion, tofind the length of the whole circumference.For, by the property of the circle, as the degreesin this angle are to the length of the base, so isthree hundred and sixty degrees to the circum-ference required.

This method of Riccioli, however, is moreingenious than accurate ; he wanted to measurethe earth, without having recourse to celestialobservations ; but the independence, to whichS 3 he