﻿160 Intelligence and Miscellaneous Articles* 



might have been expected ; and subsequent quantitative experiments 

 have confirmed this idea. The first experiments were made by ob- 

 serving black figures traced near the circumference of the disk, 

 which was illuminated solely by the rapidly recurring flashes ; and 

 it often happened that the figures, with their details, were seen quite 

 as clearly and sharply as though the disk had been stationary ; on 

 the other hand, sometimes the edges seemed blurred, as though the 

 disk had moved through a few degrees during the act of discharge. 

 The result being doubtful, the mode of experimenting was quickly 

 changed ; about fifteen narrow radial apertures were made near the 

 circumference of the disk, and the flashes and illuminated clouds 

 were observed through these openings, the disk being made to re- 

 volve as before. The distance of the eye from the apparatus was 

 about 8 inches ; and it was of course adjusted so as to obtain distinct 

 vision of the disk. The result was that sometimes the openings 

 were seen quite unchanged in appearance, but more frequently they 

 were most distinctly elongated into well-defined streaks some de- 

 grees in length. They were observed often and without difficulty ; 

 but, as further confirmation, I may add that I requested Professor 

 Joy, who was ignorant of the actual form of the aperture, to state 

 his opinion of their apparent shape while the disk was in rotation. 

 The reply was that they resembled Prince Rupert's drops, a not un- 

 fair description of the phenomena in question. Repeated estimates 

 of their size were then made with paper and pencil. Some time 

 afterward I measured the velocity which I could communicate to 

 this disk in the manner above described, by attaching to it a small 

 hollow axis through which the steel pin passed, the disk being then 

 caused to wind up a thread stretched by a small Weight. The rate 

 of rotation thus attainable was found to be about twelve revolutions 

 per second, which is a little more than I had anticipated. The ave- 

 rage size of the streaks was 9°, corresponding to a duration of ^-J-^ 

 of a second. It hence results that the duration of the flashes of 

 lightning on the occasion referred to was, in round numbers, about 

 g-^j- of a second, some of them, however, seeming to be confined to 

 smaller limits. 



I know of only a single circumstance which might militate against 

 the correctness of the above conclusion ; and it is but fair to give it 

 such weight as it may carry. Becquerel has succeeded, with some 

 difficulty, in observing a faint phosphorescence when an electric dis- 

 charge is passed through rarefied air ; and it is not absolutely impos- 

 sible that the effects observed by me were due to a cause of this 

 kind. 



This point can hereafter readily be decided by observing with a 

 revolving disk, not the distant clouds, but a sheet of white paper 

 placed so as to receive the light from the electrical flashes. — Silli- 

 man's American Journal, January 1871. 



