LET. XVI. OF THE EARTH. 26;

to be two degrees and twenty-eight minutes.He then measured their distance, in as exact; amanner as possible ; and having taken into theaccount all the turnings and windings in theroad, with the ascents and descents, he reducedit to an arc of the meridian, and found it tocontain twelve thousand, eight hundred andforty-nine chains ; and this distance, being com-pared with the difference of latitude, gave himfive thousand, two hundred and nine chains toa degree, or about sifty-seven thousand threehundred French toises.

This method will want no explanation, if thetwo places be considered as lying under thesame meridian, which indeed is nearly the cafe;for then the terrestrial and celestial arcs willexactly correspond with each other, and therelation of either of them to the whole circum-ference will be readily found. The same thingmay also be easily performed by trigonometry,when the two places lie under different meri-dians ; for if we measure the distance of any twoobjects, and take the angles which each of themmake with a third, the triangle, formed bythe three objects, will become known ; so thatthe other two sides may be as accurately deter-mined by calculation, as if they had been ac-tually measured in the same manner as the first.And by making either of these sides the base ofS new triangle, the distances of other objectsS 4 may