DENSITY OF THE EARTH. 161 



self-luminous by compression, might well render the planets 

 of this lower world unnecessary. Near the north pole, in 8 

 of latitude, an enormous opening is imagined, from which 

 the polar light visible in Aurora streams forth, and by which 

 a descent into the hollow sphere may be made. Sir Hum- 

 phry Davy and myself were repeatedly and publicly invited 

 by Captain Symmes to undertake this subterranean expe- 

 dition : so powerful is the morbid inclination of men to fill 

 unseen spaces with shapes of wonder, regardless of the 

 counter-evidence of well-established facts, or universally 

 recognised natural laws. Even the celebrated . Halley, at 

 the end of the 17th century, hollowed out the Earth in 

 lus magnetic speculations; a freely rotating subterranean 

 nucleus was supposed to occasion, by its varying positions, 

 the diurnal and annual changes of the magnetic declination. 

 It has been attempted in our own day, in tedious earnest, to 

 invest with a scientific garb that which, in the pages of the 

 ingenious Holberg, was an amusing fiction. 



The figure of the earth, its degree of solidification, 

 and its density, are intimately connected with forces 

 which act in its interior, apart from external influ- 

 ences, or the position of the planet relatively to the 

 luminous central body. We have considered the com- 

 pression or ellipticity as a consequence of the centrifugal 

 force acting on a rotating mass, and as evidencing an 

 earlier condition of fluidity in our planet: in the course 

 of the solidification of this fluid, (which some have been 

 inclined to assume to have been originally gaseous, and 

 heated to a very high degree of temperature,) an enormous 

 quantity of latent heat would have been disengaged ; and 



