202 



DENSITY OF THE EARTH. 



[Ch. XXXII. 



Ca^' 



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the globe had assumed its present form^ it was made to revolve 

 on its axis^ all matter to which freedom of motion was given 

 bj fusion, must before consolidating have been impelled 

 towards the equatorial regions in obedience to the centrifugal 

 force. Thus, lava flowing out in superficial streams would 

 have its motion retarded when its direction was towards the 

 pole, accelerated when towards the equator, or if lakes and 

 seas of lava existed beneath the earth's crust in equatorial 

 regions, as probably now beneath the Peruvian Andes, the 

 imprisoned fluid would force outwards and permanently 

 upheave the overlying rocks. The statical figure, therefore, 

 of the terrestrial spheroid (of vfhich the longest diameter 

 exceeds the shortest by about tAventy-five miles), may have 

 been the result of gradual and even of existing causes, and 

 not of a primitive, universal, and simultaneous fluidity.^ 



Experiments made with the pendulum, and observations 

 on the manner in which the earth attracts the moon, have 



shown that our planet is not an empty sphere, but, on the 

 contrary, that its interior, whether solid or fluid, has a higher 

 specific gravity than the exterior. It has also been inferred 

 from certain inequalities in the moon's motion, that there is 

 a regular increase in density from the surface towards the 

 centre, and that the equatorial protuberance is continued 



inwards ; 



that is 



to say, that layers of equal density are 



arranged elliptically, and symmetrically, from the exterior to 

 the centre. 



The mean density of the earth has been computed by 

 Laplace to be about 5^, or more than 5 times that of vfater. 

 Now the specific gravity of many of our rocks is from 2^ to 

 3, and the greater part of the metals range between that 

 density and 21. Hence some have imagined that the terres- 

 trial nucleus may be metallic— that it may correspond, for 

 example, with the specific gravity of iron, which is about 7. 

 But here a curious question arises in regard to the form 

 which materials, whether fluid or solid, might assume, if 

 subjected to the enormous pressure which must obtain at the 

 earth's centre. Water, if it continued to decrease in volume 



See ITenncssy, On Changes in Dublin, 1849 ; «ind Proc. Roy. Irish 

 Earth's Figure, &e. Journ. Geol. Soc. Acad. vol. iv. p. 337. 



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