8S AERIAL COLUMN AND AERIAL ELECTROSCOPE. 



is very considerable, in different days, and different pai ts of the 

 same day. 

 This instru- 1 8, When I first observed that phaenoraenon, it pointed out to 

 ment made j^^g ^ ^g^ g^^^j ygj.y interesting object of study: but, according 

 known imper- i ^ . a j j ' o 



fectly, to a plan of observations which 1 then formed, I was obliged to 



make many additions to my column, which required much 

 time: but the first description which I had given of that appa- 

 ratus in a paper to the Royal Society, and of its purpose, made 

 it partly known, 

 and imitatedby ^9- This accidental communication to the public was a 

 Mr.Forster. lucky circumstance ; for before J. could have time to do it my- 

 self through your Journal, a very ingenious experimental philo- 

 sopher, Mr. B. M. Forster, not knowing it precisely, imitated 

 it in a curious manner : he formed two columns, containing to- 

 gether 1500 groups of zinc and silvered paper, of the small size 

 of my first column, and having placed them horizontally, he 

 connected with each extremiity a small hell, and suspended be- 

 tween them, and very near them, a small brass hall, held by a 

 silk thread. When the apparatus was ready, he heard it chime, 

 with a sort of buzzing noise on account of the rapidity of the 

 motion of the ball. 

 Defect of his 20. This apparatus had been mentioned in Mr. Tillocb's 

 apparatus. Phil. Magazine, and having seen there its description, I spoke 

 of it in the same paper of your Journal, p. 103. But since 

 that time, having had the pleasure of making personal acquaint- 

 ance with Mr. Forster, and corresponding with him, he has 

 communicated to me, from time to time, his observations of 

 this kind of aeroscope, which, though in a different manner, 

 indicates also changes in the electrical state of the air ; for, 

 after having chimed for some time, it stops totally, then begins 

 again, andstops 5 sometimes it chimes for a moment, between 

 long intervals of silence. This is a very curious phaenomenon, 

 but there is a want of intermediary terms between the cessation 

 and return of motion. These inequalities are occasioned by 

 the insulation of the little ball, it being suspended by a silk 

 thread. Having tried what would be the effect of a greijt^r 

 distance between the bells, I found that it stopped the motiop 

 of the little ball, and I soon judged what was the cause of that 

 cessation. When there is more distance, the little ball tending 

 sensibly as much to the positive as to the negative bell, the dif- 

 ference 



