,507,] AND CONTRACTION OF THE EARTH. 209 



will be the orbit of Jupiter. Describe a cube round Jupiter's orbit; the orbit 

 including this will be the orbit of Saturn. Now inscribe in the earth's orbit 

 an icosahedron; the circle inscribed in it will be the orbit of Venus. In- 

 scribe an octahedron in the orbit of Venus; the circle inscribed in it will be 

 jMercury's orbit. This is the reason of the number of the planets." 



In this last line Kepler alludes to the five kinds of polyhedral 

 bodies, known as the only '' regular solids," corresponding to the five 

 planets, besides the earth, which were then known. 



When we consider these early views of Kepler in the light of the 

 true laws of nature, nothing so much impresses us as the highly 

 artificial character of the suggested cosmical system. But as the 

 ancients had considered the heavens to be made up of spheres of 

 crystal and glass, the contrast with actual nature implied in the sys- 

 tem proposed was much less apparent in Kepler's time than in our 

 own, because the heavens were then, through the influence of the 

 Greeks, still viewed largely as a work of art. 



The appearance in this late age of the doctrine of a terrestrial 

 tetrahedron, however, is very remarkable. The earth has been known 

 to be a spheroid differing but little from a sphere since 500 B. C, 

 and the law of gravity by which Newton established the cause of the 

 observed form of the globe has been universally recognized for over 

 220 years. iVs the earth has always been highly heated within, one 

 can not doubt that the doctrine of fluidity as applied to it is valid. 

 And the form of the surface ought not only to be spheroidal now, 

 but also throughout the whole past history of our globe. This view 

 is likewise confirmed by the figures of the other planets in space, 

 which have been carefully investigated by astronomers ever since the 

 invention of the telescope by Galileo in 1610. 



Moreover Tresca's experiments showed that under great pressure 

 all matter behaves as fluid. It is easily shown that a column of the 

 stiffest granite or steel would not stand over five miles high, without 

 crushing and vaporizing at the base. The column would begin to 

 yield when the weight became sufficient to overcome the molecular 

 forces of the material. A solid pyramid would resist crushing to 

 much greater height than a shaft of uniform diameter, but the princi- 

 ple is the same ; and no great height would be attained before even a 

 pyramid of the hardest material would begin to spread at the base 

 and flow, from its own weight. Thus there is a limit to the height 



