DRY FOG OF 17 S3. 



The heavy and constant rains which fell in June and July, 

 and the storms of wind which accompanied them, did not dissi- 

 pate it. 



Its density and partial opacity yaried in different places. In 

 Languedoc it was so dense, that the sun was not yisible at 

 altitudes below 12 ; and at greater altitudes its light was red, and 

 so subdued, that it could be looked at without inconyenience. 



The quality by which it was distinguished from common fogs 

 was its absolute dryness. Hygrometric instruments exposed in it 

 indicated the complete absence of humidity. 



One of the most remarkable circumstances, howeyer, attending 

 it was, that it appeared to be endowed with some faintly 

 luminous quality, such as might be supposed to proceed from 

 a slight degree of phosphorescence. Thus it appeared from the 

 declarations of many observers that, while it prevailed at the 

 epoch of new moon, and therefore in the total absence of moon- 

 light, the light proceeding apparently from the fog was sufficient 

 to render objects yisible at distances of two or three hundred 

 yards. 



Such being the actual phenomena, it remains to be considered 

 whether the hypothesis that the earth passed at that time through 

 the tail of a comet can be admitted to explain them. 



In the first place, it must be observed that the head of the comet, 

 if such a body were present, was not visible. This cannot be 

 explained by the supposition that the tail rendered the head 

 invisible, inasmuch as the fog did not prevent the stars being seen 

 as usual at night in all places where it prevailed. 



It has been suggested that the position of the head might have 

 been such, that it rose and set with the sun, or nearly so, and 

 could not therefore be seen in the absence of that luminary either 

 before sunrise or after sunset. But although this might be 

 admitted for a very short interval, its continuance for a month 

 would not be compatible with what is known of the motion of 

 comets. If it were a comet, the tail being generally turned from 

 the sun, the head must have been within the earth's orbit, and 

 between the earth and sun, or nearly so. The angular motion of 

 the comet must have been such as to remove it from the position 

 of inferior conjunction, in the course of a few days, after and 

 before which the head would have either risen before the sun, or 

 :tcr it, and so would have been visible. Xo such object, 

 however, was seen at or near the time of the great fog of 1783. 



Xo combination of any possible orbital motion of the comet with 

 the orbital and diurnal motion of the earth has been or can b9 

 suggested which would be compatible with the position and conti- 

 nuance of the great dry fog of 1783. It may therefore be concluded 



o2 S3 



