COMETAftY INFLUENCES. 



that that phenomenon did not arise from the immersion of the 

 earth in the tail of an unseen comet. 



The great fog of 1831 is subject to nearly the same observations, 

 and the cometaiy hypothesis is removed by nearly the same 

 reasoning. This fog was manifested during the month of August. 

 It spread over the three continents of the northern hemi- 

 sphere, commencing on the north coast of Africa on the 3rd, 

 at Odessa on the 9th, throughout France on the 10th, in the 

 United States on the 15th, and in China during the latter part of 

 the month. 



The sun's light was so enfeebled, that it could be looked at 

 without coloured or smoked glass. On the coast of Africa, the 

 sun was not visible at all at altitudes below 15 or 20 ; yet the 

 nights were so clear, that the stars were visible. Observers 

 in the north of Africa, in the south of France, in the United 

 States, and in China, reported that the disk of the sun seen 

 through the fog had the tint of azure, and in some places of 

 emerald green. 



This appearance was explained by the supposition of the well- 

 known optical illusion called "accidental colours." The fog or 

 the clouds around the solar disk, and through which the latter was 

 seen, being, like fogs and clouds in general, when seen by the 

 transmitted light of the sun, reddish, the white disk of the sun 

 seen in juxta-position with them would, by the mere effect of 

 contrast, appear to be bluish or greenish, according to the tint of 

 red transmitted by the surrounding clouds.* 



Like the great fog of 1783, this fog seemed to have a proper 

 light. During its prevalence there was, strictly speaking, no 

 nocturnal darkness. During the month of its prevalence there 

 was light enough at midnight to read the smallest written or 

 printed characters. This fact was reported equally by observers 

 in places the most distant, as in Italy, Prussia, Siberia, &c. 



Since twilight ceases when the depression of the sun below the 

 horizon exceeds 18, and since at these places, in August, the 

 depression considerably exceeds that limit, it is evident that the 

 light thus observed could not have been common twilight. 



Whatever may be the explanation of this phenomenon, that of 

 the immersion of the earth in the tail of a comet is overthrown 

 completely by the fact that the fog, though extensively spread, 

 was not continuous, much less uniform. Some parts of the 

 European continent were altogether or nearly free from it, and 

 in other parts it was developed in very different degrees. The 

 times of its continuance in different places also varied much and 



* See Lardner's ( 'Hand-Book of Natural Philosophy" (1159). 



