288 Professor W, Thomson, [May 18, 



parts of the lecture-room, while insulated spirit lamps connected with 

 the positive and negative conductor of an electrical machine, burned 

 on the two sides. The speaker observed the indications on the portable 

 electrometer ; but the potentials thus measured, were seen by the audience 

 marked on the scale by the spot of light ; the reflecting electrometer 

 being kept connected with the portable electrometer in all its positions, 

 by means of a long fine wire. It was found that when the burning 

 match was on one side of a certain surface dividing the air of the 

 lecture- room, the potential indicated was positive, and on the other 

 side negative. 



The water-dropping collector constructed for the self-registering 

 apparatus to be used at Kew, had been previously set upon the roof 

 of the Royal Institution, and an insulated wire (Beccaria's " Deferent 

 Wire *') led down to the reflecting electrometer on the lecture-room 

 table. The electric force in the air above the roof was thus tested 

 several times during the meeting ; and it was at first found to be, as it 

 had been during several days preceding, somewhat feeble positive 

 (corresponding to a feeble negative electrification of the earth's surface, 

 or rather housetops, in the neighbourhood). This was a not unfrequent 

 electrical condition of days, such as these had been of dull rain, with 

 occasional intervals of heavier rain and of cessation. The natural 

 electricity was again observed by means of the reflecting electrometer 

 during several minutes near the end of the discourse ; and was found 

 instead of the weak positive which had been previously observed, to be 

 strong positive of three or four times the amount. Upon this the 

 speaker quoted* an answer, which Prior Ceca had given to a question 

 Beccaria had put to him " concerning the state of electricity when the 

 weather clears up." *' ' If, when the rain has ceased (the prior said 

 to me) a strong excessive^ electricity obtains, it is a sign that the 

 weather will continue fair for several days ; if the electricity is but 

 small, it is a sign that such weather will not last so much as that 

 whole day, and that it will soon be cloudy again, or even will again 

 rain.' '* The climate of this country is very different from that 

 of Piedmont, where Beccaria and his friend made their observations, 

 but their rule as to the " electricity of clearing weather,'* has been 

 found frequently confirmed by the speaker. He therefore considered, 

 that although it was still raining at the commencement of the meeting, 

 the electrical indications they had seen gave fair promisej for the re- 

 mainder of this evening, if not for a longer period. There can be no 

 doubt but that electric indications, when suflftciently studied, will be 

 found important additions to our means for prognosticating the weather ; 



* From Beccaria's first letter " On Terrestrial Atmospheric Electricity during 

 Serene Weather." — Garzegna di Mondovi, May 16, 1775. 



t i.e. vitreous, or positive. 



X At the conclusion of the meeting it was found that the rain had actually- 

 ceased. The weather continued fair during the remainder of the night, and 

 three or four of the finest days of the season followed. 



