THE DENSITY OF THE EARTH. 35 



Ttit torsion-balance. Cavendish (according to Baily's calcula- 

 tion) 5*44:8 



Reich, 1838 5-440 



Bailv, 1832 5-660 



Reich, 1847-1850 5-577 



A far more important result in reference to the density of 

 the earth than that obtained by Baily (1842) and Reich 

 (1847-1850) has been brought out by Airy's experiments 

 with the pendulum, conducted with such exemplary care in 

 the Mines of Harton, in the year 1854. According to these 

 experiments the density is 6*566, with a probable error of 

 0*182 (Airy, in the Philos. Transact, for 1856, p. 342). A 

 slight modification of this numerical value, made by Pro- 

 fessor Stokes on account of the effect of the rotation and el- 

 lipticity of the earth, gives the density for Harton, which 

 lies at 54° 48' north latitude, at 6*565, and for the equator 

 at 6-489. 



The mean of the two last results gives 5-62 for the density 

 of the earth (taking that of water as 1), and consequently 

 much more than the densest finely granular basalt, which, 

 according to the numerous experiments of Leonhard, varies 

 from 2-95 to 3-67, and more than that of magnetic iron (4-9 

 to 5*2), and not much less than that of the native arsenic of 

 Marienbe'rg or Joachimsthal. "We have already elsewhere 

 observed (Cosmos, vol. i., p. 167) that from the great distribu- 

 tion of secondary and tertiary formations, and of those up- 

 heaved strata which constitute the visible continental part 

 of our earth's surface (the Plutonic and volcanic upheavals 

 being scattered in the form of islands over a small area of 

 space), the solid portion of the upper part of the earth's crust 

 possesses a density scarcely reaching from 2-4 to 2 -6. If we 

 assume with Rio;aud that the relation of the solid to the fluid 

 oceanic surface of our globe is as 10 : 27, and if further we 

 consider that the latter has been found by experiments with 

 the sounding-lead to extend to a depth of 27,700 feet, the 

 whole density of the upper strata, which underlie the dry 

 and oceanic surfaces, scarcely equals 1*5. The distinguished 

 geometrician Plana has correctly observed that the author of 

 the Mecanique Celeste was in error when he ascribed to the 

 upper stratum of the earth a density equal to that of granite, 

 which, moreover, he estimated somewhat highly at 3, which 

 would give him 10*047 for the density of the centre of the 

 earth.* This density would, according to Plana, be 16*27 



* Laplace, Mecanique Celeste, e'd. de 1846, t. v., p. 57. The mean 



