Entretiens familiers sur l'astronomie, avec des chapitres sur la géographie et l'navigaton . RoyalSociety pour essayer la deuxième méthode. En conséquence, l'Accord-il s'en alla en 1774 à un très haut mountaincalled Schehallien, dans le Perthshire, et il hemeasured l'inclinaison ou la déformation des theplumb-ligne de chaque côté de la montagne. Selon la théorie de la gravitation,-suppos-ing la terre d'être une sphère parfaite, - l'leadat la fin de la ligne doit pointer directement au-ward le centre C de la terre (Fig. 10) si themountain n'y touchez pas ; et si le fil à plomb est pris à deux endroits un certain distanceapart et inclina sa

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Familiar talks on astronomy, with chapters on geography and navigaton . RoyalSociety to try the second method. Accord-ingly, he went in 1774 to a very high mountaincalled Schehallien, in Perthshire, and there hemeasured the inclination or deflection of theplumb-line on each side of the mountain. Ac-cording to the theory of gravitation, —suppos-ing the earth to be an exact sphere, — the leadat the end of the line should point directly to-ward the centre C of the earth (Fig. 10) if themountain did not disturb it; and if the plumb-line is taken to two places a certain distanceapart and its inclination measured by means ofcelestial observations, it is easy to find out howmuch the lines D F, D F, will incline toward The Earttis Density. 145 each other when no mountain is between them.This being known, Maskelyne then made twoobservations, one on each side of Schehallien, and found that in this case the inclination, in-stead of being from D to F, and from D to F, on each side, was from E to F, and from E toF, because the mountain attracted the lead to- E D tf E. ward itself on either side. So the deflectionE F D, or E F D, through which the plumb-line was drawn from the perpendicular showedthe difference between the attraction of thewhole earth and the attraction of the mountain.See Fig. 10; A B is the surface of the earth;D C D the angle formed by two plumb-lines10 146 Familiar Talks on Astronomy, etc. pointing to the centre of the earth ; and EGEthe angle formed by the two plumb-lines, whendrawn aside by the mountain, M. Dr. Hutton, the celebrated mathematician, calculated the size and weight of Schehallien.1This he did by surveying it and measuring itin every direction, and then taking pieces of thedifferent rocks it contained and weighing themcarefully. When this was done it was foundthat the mountain pulled half as strongly incomparison to its size as the earth did for itssize. This showed that the materials in themountain were half as heavy as the average ofthose in

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