V 



TENSITY OF THE EARTH. 163 



scarcely 1 '6, it follows, that the elliptical unequally compressed 

 layers of the interior must greatly increase in density towards 

 the centre, either through pressure or owing to the hetero- 

 geneous nature of the substances. Here, again, we see that 

 the vertical, as well as the horizontally vibrating pendulum, 

 may justly be termed a geognostical instrument. 



The results obtained by the employment of an instrument 

 Df this kind, have led celebrated physicists, according to the 

 difference of the hypothesis from which they started, to adopt 

 entirely opposite views regarding the nature of the interior of 

 the globe. It has been computed at what depths, liquid or 

 even gaseous substances would from the pressure of their own 

 superimposed strata, attain a density exceeding that of platinum 

 or even iridium ; and in order that the compression which has 

 been determined within such narrow limits might be brought 

 into harmony with the assumption of simple and infinitely 

 compressible matter Leslie has ingeniously conceived the 

 nucleus of the world to be a hollow sphere, filled with an as- 

 sumed " imponderable matter, having an enormous force of 

 expansion." These venturesome and arbitrary conjectures 

 have given rise, in wholly unscientific circles, to still more 

 fantastic notions. The hollow sphere has by degrees been 

 peopled with plants and animals, and two small subterranean 

 revolving planets Pluto and Proserpine were imaginatively 

 supposed to shed over it their mild light ; as, however, it 

 was further imagined that an ever-uniform temperature 

 reigned in these internal regions, the air, which was made 

 self-luminous by compression, might well render the planets 

 of this lower world unnecessary. Near the North Pole, at 

 82 latitude, whence the polar light emanates, was an enor- 

 mous opening through which a descent might be made into 

 the hollow sphere, and Sir Humphrey Davy and myself were 

 even publicly and frequently invited by Captain Symmes to 

 enter upon this subterranean expedition: so powerful is the 

 morbid inclination of men to fill unknown spaces with shapes 

 of wonder, totally unmindful of the counter- evidence furnished 

 by well-attested facts and universally acknowledged natural 

 laws. Even the celebrated Ilalley, at the end of the seven- 

 teenth century, hollowed out the Earth in his magnetic specu- 

 lations ! Men were invited to believe that a subterranean 

 freely-rotating nucleus occasions by its position the diuinal 



v. 2 



