ON THE ORIGIN OF THE PLANET AEY SYSTEM. 161 



more difficultly volatile constituents as fog or cloud. 

 This cooling can only, of course, be regarded as com- 

 parative ; their temperature is probably, even then, 

 higher than any temperature attainable on the earth. 

 If now the upper layers, freed from the heavier 

 vapours, sink down, there will be a space over the 

 sun's body which is free from cloud. They appear 

 then as depressions, because about them are layers of 

 ignited vapours as much as 500 miles in height. 



Violent storms cannot fail to occur in the sun's 

 atmosphere, because it is cooled on the outside, and 

 the coolest and comparatively densest and heaviest 

 parts come to lie over the hotter and lighter ones. 

 This is the reason why we have frequent, and at times 

 sudden and violent, movements in the earth's atmos- 

 phere, because this is heated from the ground made 

 hot by the sun and is cooled above. With the far 

 more colossal magnitude and temperature of the sun, 

 its meteorological processes are on a far larger scale, 

 and are far more violent. 



We will now pass to the question of the perman- 

 ence of the present condition of our system. For a 

 long time the view was pretty generally held that, in 

 its chief features at any rate, it was unchangeable. 

 This opinion was based mainly on the conclusions at 

 which Laplace had arrived as the final results of his 

 long and laborious investigations, of the influence of 

 n. M 



