124 LAMBERT: CONSTITUTION OF THE EARTH 



well in the past, and is very probably valid, if we understand it 

 in the above sense, that for large stresses, long continued, the 

 earth acts on the whole like a fluid body. 



The hypothesis of fluidity did not have the field all to itself, 

 and as a matter of curiosity I will mention some of the more ex- 

 travagant of the competing notions. Some of you may have 

 heard of "Symmes's hole,"^ an opening at both poles, admitting 

 to several layers of habitable spheres in the interior of the earth. 

 Symmes apparently put forward his idea in all seriousness and 

 asked for the fitting out of polar expeditions to find the entrance 

 to this unexplored and desirable territory. Equally strange ideas 

 have been put forward by men of higher scientific standing than 

 Captain Symmes. The astronomer Halley supposed the in- 

 terior of the earth to be hollow, with inner spheres much like 

 Symmes's, only with no hole to give access to them. These 

 spheres were assumed to be magnetic, their rotation at a slightly 

 different rate from that of the outer sphere causing the variation 

 of the magnetic elements. A contemporary of Symmes, not as 

 well known as Halley, conceived the idea of a magnetic planetoid 

 within the earth. Benjamin Franklin, usually level-headed, 

 supposed the interior of the earth to be filled with compressed 

 air.^ 



Let us leave now these airy realms of fancy and consider what 

 we know of the density of the earth. The rock on the surface 

 is directly accessible. The extremes of rock density are about 

 3.3 and 2.1; the mean for the earth's surface as a w^hole may be 

 put at 2.6 to 2.8. We can judge of the density below the levels 

 accessible to us only by the mechanical effects of the matter 

 of these inaccessible regions. One mechanical effect is the at- 



^ The Symmes Theory of Concentric Spheres, demonstrating that the earth is hollow, 

 habitable within, and widely open at the Poles. Compiled by Americus Symmes from 

 the writings of his father, Capt. John Cleves Symmes. Published at Louisville, Ken- 

 tucky, in 1878. Capt. Symmes served with credit in the War of 1812. 



^ Convenient summaries of early speculations about the earth's interior, with 

 references to the literature of the subject, will be found in Thiene, Temperatur 

 und Zustand des Erdinnern (Leipsic, 1907), Chapter i; and in Gunther, Lehrbuch 

 der Geophysik, Vol. i. Thiene's version of Halley 's ideas is, however, erroneous. 

 See Phil. Trans., 1692, p. 563. 



