68 PROFESSOR W. THOMSON ON THE 



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 insensi- 

 ble by any kind of observation (that of eclipses included) unassisted by powerful 

 telescopes. We may be confident, then, that the gradual augmentation 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 observation. This leaves for the specu- 

 lations 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 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 explana- 

 tion of the appearance and disappearance of bright stars, and of the strange va- 

 riations of brilliancy of others which have caused so much astonishment.* 



The amount of matter, drawn by the Sun in any time from surrounding space, 

 would be such as in 47^ years to amount to a mass equal to that of the earth. 

 Now there is no reason whatever to suppose that 100 times the earth's mass 

 drawn in to the Sun, would be missed from the zodiacal light (or from meteors 

 revolving inside the orbit of Mercury, whether visible as the " zodiacal light" or 

 not) ; and we may conclude that there is no difficulty whatever in accounting for 

 a constancy of solar heat during 5000 years of time past or to come. Even physi- 

 cal astronomy can raise no objection by shewing that the Sun's mass has not ex- 



* The star which Mr Hind discovered in April 1848, and which only remained visible for a few 

 weeks, during which period it varied considerably in appearance and brightness, but was always of 

 a " ruddy" colour, may have not experienced meteoric impact enough to make its surface more 

 than red hot. 



