Oil the Maintenance of Energy. 17 



the heavier ones. This would occasion a decrease of density 

 at the outer parts of the nebula. Now the formation of the 

 planets can be explained in somewhat the same way as 

 before. The sum total of the rotary motion now existing in 

 the planetary system must have been in the nebula, and as 

 the volume contracted by gravitation towards the centre, 

 its velocity of rotation would constant!}^ increase, so tliat at 

 last the centrifuoal force due to rotation would balance the 

 attractive force of gravity. 



The orbits of all the larger planets are approximately in 

 the plane of the ecliptic, and a planet may be regarded as 

 having been formed from masses of the nebula thrown off by 

 centrifugal agencies. On assuming a separate existence from 

 the parent nebula, we must consider its development to have 

 been analagous to that of the sun. There would be a 

 continual falling together of the particles composing it to the 

 centre of gravity of the mass. This condensation would be 

 accompanied by the generation of heat through friction 

 between the particles, and a further differentiation of matter 

 would be occasioned So that the matter com])osing 

 satellites would be of less density. The forces at work were 

 exactly the same as those in the sun, but different in degree. 

 We may draw then the following conclusions concerning the 

 earth ; that it was never wholly in a plastic or a gaseous 

 state, that its centre was relatively cool and solid and tliat 

 the temperature gradually increased towards the surface ; 

 that tln-ough loss of heat by radiation into space a solid 

 crust was formed, and that the cooler matter in the iiiterior 

 became gradually heated through conduction, and the more 

 volatile constituents continually assumed a liquid and 

 vapourous condition. Now these vapours would be obliged 

 to find some outlet. The cooled crust at the surface would 

 crack just in the same way that a boiler full of water, 

 subjected to intense heat and without a safety valve, would 

 crack. Now terrestrial volcanoes ai'e built up along lines of 

 fissure in the earth's crust and almost all the active ones are 

 situated on rising areas. Nearly all can be shown to be 

 thrown up along three well-marked lands or great fissures 

 and the branches proceeding from them. They traverse the 

 surface of the o-lobe in sinuous lines with a o-eneral north and 

 south direction and the branches often appear to form 

 connections between the great bands. The first and most 

 important of these bands is nearly 10,000 miles in length. 

 It stretches from near the Arctic Circle at Behring's Straits, 



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