Page:Popular Science Monthly Volume 17.djvu/308

294 Peru. Aided by La Condamine, he observed the variation in the plummet-line due to the influence of Chimborazo, and he noted the rate of the pendulum's movement on the volcanic mountain of Pichincha (which is equal in height to Mont Blanc), and at the sea-level. Unfortunately, the imperfection of his instruments, the rigor of the climate, and the violence of the winds, prevented the two French astronomers from attaining great precision in these observations. The effects they had expected to see confirmed were found to be much less marked than they had anticipated, and Bouguer therefore believed that the volcanic mountains of Peru were hollow and internally simply huge caverns. A repetition of his experiments, with the care required in researches of such a delicate nature, would determine the question whether the unsatisfactory character of his results was due to errors of observation, or if it was a case similar to that of the Himalayan chain.

Bouguer's method was employed successfully in 1774 by the celebrated English astronomer Maskelyne. He chose for his experiments Mount Schiehallion in Scotland. This mountain is wholly isolated. Its geologic constitution is known, and its form is not very irregular; the calculations were thus simplified. By observations of the stars that passed near his zenith Maskelyne first determined the latitudes of two stations, one to the south and the other to the north of the mountain—the distance between them being 1,330 metres. The difference in the two astronomical latitudes was found to be 43″ instead of 54″. 6, as shown by the measured distance. The excess of 11″.6 represented the sum of the attractive force exerted by Schiehallion on its opposite sides. It then remained to ascertain the volume of the mountain, its exact configuration, density, and total weight, and by the aid of these factors to determine the theoretic sum of the attraction it exerted on the plummet of the two stations. The geologist Hutton was intrusted with this work, which occupied him three years. The result of Lis calculations was, that the deviation observed would be explained by supposing the mean density of the mountain to be to that of the earth as 5 is to 9. Hutton first adopted for the density of Schiehallion the number 2·5—about the density of quartzose sandstone; according to this the mean density of the globe was 4·5. He afterward modified these figures, taking 3·0 for the density of the mountain and 5·4 for that of the earth. The geological study of this mountain, undertaken subsequently by Playfair and Lord Webb-Seymour, showed the density of its component rocks to be intermediate between these two estimates, from which it would appear that the earth's density is 4·7.

These experiments were not supplemented by observations of the