196 O. C. JONES, GEORGE D. HUBBARD Vol. XXII, No. 7 



The plumb li?ie^~^° has been used near mountains to estimate 

 the mass of the earth. The deflection from the vertical, 

 due to the attraction of the mountain, is measured. The mass 

 of the mountain is then calculated, and from these data the 

 mass of the earth is calculated. By this method Maskelyne, 

 in 1775, secured a mean density of 5. and James, in 1854, 

 obtained 5.32. 



The pendulum^~^^ was first used by Bouguer. The number 

 of swings of the pendulum at the surface are compared with the 

 number on a mountain, and the comparative attraction worked 

 out. Carlini, 1821, obtained a mean density of the earth of 

 4.84; Airy, 1854, obtained 6.57; Pechman, 1865, 6.13; Menden- 

 hall, 1880, 5.77; Sterneck, 1883, 4.77, and in 1885, 6.77; Preston, 

 1892, 5.13. 



The Cavendish method. — In this experiment two small balls 

 are placed at opposite ends of a wire and their movement 

 measured when attracted by a large ball of known mass. A 

 comparison is then made between the attraction of the earth 

 and the attraction of the ball. Cavendish, in 1798, found 

 5.45 to be the mean density of the earth; Bailey, 1843, secured 

 5.67; Reide, 1852, 5.58; Corme and Bailie, 1878, 5.5; Wilsing, 

 1889, 5.56; Preston, Bays and Braun, in 1895 and 1896, all 

 secured 5.53; in 1902, Burgess obtained 5.55. 



The chemical balance has been successfully used by determin- 

 ing the attraction of a known mass placed above or below the 

 scale, and comparing with mass of earth. By this method 

 Poynting, 1891, secured a specific gravity of the earth of 5.49; 

 and Richarz and Krigar Menzel in 1898 obtained 5.51. 



Tides. — The evidence from tides is very simple yet very con- 

 clusive. If the earth is liquid with a crust or lacking in rigidity, 

 then the entire crust will yield for a tide; but if it is rigid — as 

 rigid as steel — there will be some rock tide, but the tidal force 

 will be largely consumed in raising tides in the more responsive 

 hydrosphere. Calculations on the height of tides by Darwin^^ 

 and others shows that the tides are exactly what are to be 

 expected on an earth of the rigidity of steel. Prof. Millikan 

 has, however, measured tides in the rocks. 



^New International Encyclopoedia. 

 'oQsmond Fisher: Physics of Earth's Crust. 

 '^Darwin: Tides. 



