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



of the sun's light to an observer on tlie earth. 



Perhaps the more prevalent idea, at the present dny, is that the cor 

 rays and tufts of hght, is a phenomenon of diffraction pr(jdnced by th 

 the s\m's rays along the denticulated ed^re of the moon. This theoi 

 of plausibility, but it is entirely inadequate to account for the great e 

 coronal rays. The fringes produced by the diffraction of light in its ] 

 the edge ot" a body appear to the eye of the observer to extend but a s 

 distance from the edge. This would be more s-trikingly true in the c 



kind permanently connected with the sun, or is made up of muteria] 

 proceeding immediately from the sun. To the large body of indtrt 



" '■' an examination of the photo- 



After the completion of my first set of experiments on tlie 

 duration of the discharge of a Leyden jar, I became anxious to 

 make some measurements of the duration of a flash of ordinary 

 lightning, which may be considered as equivalent to the dis- 

 charge of an immense jar with an enormous striking-distance. 

 The results of Feddersen have shown that the duration of the 

 discharge is increased by an addition to the size of the jar, as 

 "Well as by augmentation of the striking-distance, and as both 

 these quantities are so large with a flash of lightning, it was 

 reasonable to expect that the duration of its discharge would 

 be prolonged in some corresponding ratio. During the violent 

 thunder-storm of last August, which occurred in the evening, 

 I happened to be at a house commanding an unobstructed y-' 

 of the horizon, and this circumstance take 



with 

 the frequency' and proximity of the electrical discharges, in- 

 duced me, although entirely unprovided with apparatus, to at- 

 tempt a measurement of their duration. A circular disc, five 

 inches in diameter, was hastily cut from white cardboard, while 

 a steel shawl-pin sei-ved as an axis, on which it was made to 

 revolve by constantly striking its edge tangentially with the 

 nght hand, the pin being held in the left. The maxvmum veloc- 

 ity attainable in this way was always employed. The general 

 indications at the time were that the rate thus obtained was 

 considerably more uniform than might have been expected, 

 and subsequent quantitative experiments have confirmed this 



