Cometary Bodies and Saturn's Rings. 743 



Assuming the secular cooling of the sun to be continuous, 

 the liquefaction and final solidification of his outward parts 

 would follow in natural sequence in accordance with common 

 experience of cooling bodies, while the central parts would 

 remain in their primitive gaseous condition. From strict 

 analogy, it may justly be inferred that all the planetary 

 bodies have gone through the same stages of cooling as those 

 outlined in the instance of the central body. 



The notion that the earth and, inierentially, the other 

 planets are solid bodies throughout, finds no support from a 

 reasonable consideration of the constituents of the earth's 

 crust, so far as they are accessible to observation. The late 

 distinguished Professor of Geology in Oxford University 

 (Sir Joseph Prestwichj, in his classical woik on Chemical, 

 Physical, and Stratigraphical Geology, has clearly demon- 

 strated from the uplift of continental areas and mountain 

 chains, the welling out of basaltic lavas over many thousand 

 square miles of surface and of great thickness, that a com- 

 paratively thin crust enveloping a fluid interior is a necessary 

 condition to satisfy the requirements of geologists and 

 physicists. More significant still is the succession of foldings 

 of the earth's crust and stratigraphic contortions of small 

 curvature, both of which features indicate a thickness of 

 solid crust less than twenty-five miles. How far the im- 

 prisoned gases at the centre of the earth and the aqueous 

 vapours near the surface may have contributed respectively 

 to produce these geological changes, it is unnecessary now 

 to discuss, but in the instance of the moon, which has neither 

 water nor an atmosphere, the evidence of intense volcanic 

 action manifested on its surface can only be accounted for 

 by the ejective force of the gaseous substances in its interior, 

 similar to that by which the incandescent gases from the 

 surface of the sun are projected. 



The fine series of photographic enlargements of the moon 

 executed by MM. Loewy and Puiseux, of the Paris Obser- 

 vatory, show the greater part of its surface, from the equator 

 to the poles, covered with extinct volcanoes in every stage 

 of formation, similar to those on the terrestrial globe. Some 

 of these volcanoes are twelve thousand feet in height, with 

 their craters upwards of forty miles in diameter, and are 

 striking evidence of the immense repulsive force which pro- 

 duced them. 



It has for a long time been considered on good evidence 

 that the planetoids between the orbits of Mars and Jupiter 

 (now numbering more than 600) are the fragments of a large 

 planet which had formerly revolved in an orbit about the 



