16 0. K Rood on the duration of Flashes of Lightning. 



idea. The first experiments were made by observing black 

 figures traced near the circumference of the disc, 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 disc had been stationary ; on 

 " - - -^ ^ ■• - ^ -' ^ as though 



the other hand, sometimes the edges seemed blurred, i 

 the disc had moved through a few degrees during the i 

 discharge. The result being doubtful, the mode of experi- 

 menting was quickly changed; about fifteen narrow radial 

 apertures were made near the circumference of the disc, and 

 the flashes and illuminated clouds were observed through these 

 openings, the disc being made to revolve as before. The dis- 

 tance of the eye from the apparatus was about eight inches, 

 and It was of course adjusted so as to obtain distinct vision of 

 the disc. 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 

 degrees m length. They were observed often and without dif- 

 ficulty, but as farther confirmation I may add that 1 requested 

 Prof Joy, who was ignorant of the actual form of the aper- 

 ture, to state his opinion of their apparent shape while the disc 

 was m rotation. The repljr was that they resembled Prince 

 Kupert s drops— a not unfair description of the phenomena in 

 question. Eepeated estimates of their size were then made 

 with paper and pencil. Some time afterward I measured the 

 velocity which I could communicate to this disc in the manner 

 above described, by attaching to it a small hollow axis through 

 which the steel pm passed, the disc 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 

 average size of the streaks was 9°, corresponding to a duration 

 ot ^i^ of a second. It hence results that the duration of the 

 flashes of lightning on the occasion referred to was in round 

 numbers about -,U oi a second, some of them, however, seem- 

 ing 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 

 lair to g^ve it such weight as it may carry. Becquerel has 

 succeeded with some difficulty in observing a faint phospho- 

 rescence when an electric discharge is passed through rarefied 

 air, and it is not absolutely impossible that the effects observed 

 by me were due to a cause of this kind. 



Ihis pomt can hereafter readily be decided by observing with 

 a revolving disc, not the distant clouds, but a sheet of white 

 paper, placed so as to receive the light from the electrical flashes. 



Columbia CoUege, Nov. 10th, 1870. 



