DENSITY OP THE EARTH. 81 



As soon as the measurements of degrees in very different 

 latitudes had manifested that the interior of the earth 

 cannot be assumed to be of uniform density, since their 

 results showed an ellipticity much inferior to that assumed 

 by Newton ( T -}-o), and greatly exceeding -yj-g-, the amount 

 corresponding to the hypothesis of Huygens of the whole 

 attraction being concentrated at the centre of the earth, 

 the connection of the amount of ellipticity with the " law of 

 density" in the interior of the terrestrial spheroid could not 

 but become an important subject for analytical investigation. 

 Theoretical speculations on gravity led early to the conside- 

 ration of the attraction which might be exercised by moun- 

 tain masses rising like banks from the dry bottom of the 

 atmospheric ocean. Newton had already examined, in his 

 " Treatise of the System of the World in a popular way, 

 1728," how much a mountain 2500 Paris feet high, and 

 having a diameter of 5000 such feet, would deflect the 

 plumb line from its vertical direction. It was probably this 

 examination which gave occasion to the not very satisfactory 

 experiments of Bouguer at Chiinborazo,( 25 ) and of Maske- 

 lyne and Hutton at Schehallien in Perthshire, near Blair 

 Athol ; to the comparison of the lengths of the seconds 

 pendulum on the sea-shore and at 6000 Erench feet above 

 the sea (at the Hospice on Mount Cenis by Carlini, and 

 near Bourdeaux by Biot and Mathieu) ; and lastly, to the 

 delicate and alone decisive experiments of Reich and Baily, 

 made with the ingeniously -devised " Torsion Balance," 

 invented by John Mitchell, ( 26 ) and transmitted through 

 Wollastou to Cavendish. 



These three different modes of determining the density of 



