Mechanical Energies of the Solar System. 415 



that which was determined above according to Mr. Waterston's 

 form of the theory, and must consequently amount to 3800 lbs. 

 annually per square foot. If, as was before supposed, the density 

 of the deposit is the same as that of water, the whole surface 

 would be covered annually to a depth of 60 feet, from which the 

 sun would grow in diameter by a mile in 88 years. It would 

 take 4000 years at this rate to grow a tenth of a second in 

 apparent diameter, which could scarcely be perceived by the 

 most refined of modern observations, or 40,000 years to 

 grow 1", which would be utterly insensible by any kind of ob- 

 servation (that of eclipses included) unassisted by powerful 

 telescopes. We may be confident, then, that the gradual aug- 

 mentation of the sun^s bulk required by the meteoric theory to 

 account for this heat, may have been going on in time past 

 during the whole existence of the human race, and yet could 

 not possibly have been discovered by observation, and that at 

 the same rate it may go on for thousands of years yet without 

 being discoverable by the most refined observations of modern 

 astronomy. It would take, always at the same rate, about 

 2,000,000 years for the sun to grow in reality as much as he 

 appears to grow from June to December by the variation of the 

 earth^s distance, which is quite imperceptible to ordinary observa- 

 tion. This leaves for the speculations of geologists on ancient 

 natural history a wide enough range of time with a sun not sensibly 

 less than our present luminary : still more, the meteoric theory 

 affords the simplest possible explanation of past changes of climate 

 on the earth. For a time the earth may have been kept melted by 

 the heat of meteors striking it. A period may have followed 

 when the earth was not too hot for vegetation, but was still kept, 

 by the heat of meteors falling through its atmosphere, at a much 

 higher temperature than at present, and illuminated in all regions, 

 polar as well as equatorial, before the existence of night and day. 

 Lastly ; although a very little smaller, the sun may have been 

 at some remote period much hotter than at present by having a 

 more copious meteoric supply. 



A dark body of dimensions such as the sun, in any part of 

 space, might, by entering a cloud of meteors, become incandescent 

 as intensely in a few seconds as it could in years of continuance 

 of the same meteoric circumstances ; and on again getting to a 

 position in space comparatively free from meteors, it might almost 

 as suddenly become dark again. It is far from improbable that 

 this is the explanation of the appearance and disappearance of 

 bright stars, and of the strange variations of brilliancy of others 

 which have caused so much astonishment*. 



* The star which Mr. Hind discovered in Aprd 1848, and which only 

 remained visible for a few weeks, during which period it varied coimiderably 



