294 THE POPULAR SCIENCE MONTHLY. 



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 Pichin- 

 cha (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 as- 

 tronomers 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 cele- 

 brated 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 simj)lified. 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 

 jflummet 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 exjDlained 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 47. 



These experiments were not supplemented by observations of the 



* M. Saigey has shown that, by selecting from Bouguer's observations those which 

 appear to have been made under favorable conditions, and by calculating the force of 

 the attractions in a more exact manner, the density of the earth is found to accord with 

 Maskelyne's estimate of it. 



