VOL. LXIV.] PHILOSOPHICAL TRANSACTIONS. ' 505 



§ 6. Experiments and Observations on the Electricity of Fogs, &c. — 1771, 

 Nov. 14, half past 8, A. m. I find a fog, not very thick, pretty strongly electri- 

 fied. The balls separate full half an inch. They keep stationary, there being 

 little or no wind. 



Dec. 2, half past 8, a. m., a fog, moderately thick, is strongly electrified. 

 The balls diverge half an inch ; but when they are brought near the building, 

 they close, and open again on removing them. The mercury in the thermometer 

 is 1 5 degrees above the freezing point. 



Dec. 18, half past 4, p. m., a moderately thick fog is strongly electrified soon 

 after its appearance. The balls diverge full half an inch, and regularly close at 

 the approach of excited wax. The wind is troublesome, but the balls keep their 

 distance, and at intervals very well admit trying the experiment. 



1772, Jan. 5,' a fog is strongly electrified positively. The balls diverge full 

 half an inch. The air is sharp, and frosty. 



Jan. 13, 9 o'clock, a. m., a fog, not very thick, is strongly electrified posi- 

 tively. The mercury in the thermometer is 7-5- degrees above the freezing point. 

 There is little or no wind. 



Jan. 18, 10 o'clock, a.m. The air is pretty strongly electrified by a fall of 

 snow. 



From the small number of experiments I have been able to make on the elec- 

 tricity of the atmosphere, I cannot help being of opinion, that fogs are much 

 more strongly electrified in, or immediately after, a frost, than at other times; 

 and that the electricity in the fogs is often the strongest, soon after their appear- 

 ance. I also now hold it for a certain rule, that whenever there appears a thick 

 fog, and the air is at the same time sharp and frosty, that fog is strongly elec- 

 trified positively. Though rain may not be an immediate, yet I am inclined to 

 think it is by no means a very remote consequence of electricity in the atmos- 

 phere; and, from the trifling observations I have had an opportunity to make on 

 that subject, I have not failed to find that in 2 or 3 days after I have discovered 

 the air to be strongly electrified, especially if that electricity continued for as long 

 a time, we have had rain, or other falling weather, and I incline to believe, more 

 plentifully in proportion to the strength and continuance of the electricity; if 

 not rain, snow, &c. according to the state of the atmosphere, with respect to 

 heat and cold. If electricity be not a cause, I think it at least a prognostic, of 

 falling weather. 



XLII. A Letter from David Macbride, accompanying a Letter from Mr. Simon 

 to Dr. Macbride, concerning the Reviviscence of some Snails preserved many 

 Years in Mr. Simons Cabinet, p. 432. 



In Mr. Simon's letter of the 26th of November, he mentions a particular 



