106 Dr. H. H. Hoffert on 



Experiments Eos. 1, 2, 3, 6, 14A', 17A, and 22 were in- 

 terrupted by divers accidents and never completed ; in 

 No. 19 there is confusion in the notes, making the sign of 

 the deflexion doubtful ; Nos. 21, 23, and 24 give values of 

 p from 2 per cent, to 10 per cent, out, due to some error in 

 the record of resistances used. This accounts for all the 

 experiments begun. 



The average of all the above is "98622; without No. 27 

 which differs about twice as much from the mean as any other 

 observation, the average is *98634. The great divergence of 

 No. 27 is in itself reason enough for giving it less weight ; 

 but in addition, the chronograph sheet shows that the speed 

 here was verj' irregular, increasing, decreasing, and increasing 

 again ; this is the only occurrence of such irregularity. 

 Therefore, giving it about one third weight, we find as the 

 most probable value 



1 B.A. Unit = -9863 ohm. 



A determination of the "Mercury Unit" was recently 

 made by Messrs. Hutchinson and Wilkes (Johns Hopkins 

 University Circulars, May 1889 ; Phil. Mag. July 1889) 

 who found the value to be *95341. Taking this with the 

 above number for the B.A. unit, we have as the length of the 

 mercury column corresponding to the ohm, 



106*34 centims. 



XV. Intermittent Lightning-Flashes. By H. H. Hoffert, 

 D.Sc, A.R.S.M., Demonstrator of Physics at the Normal 

 School of Science and Royal School of Mines *. 



[Plate IV.] 



ri^HE storm which passed over London on the evening of 

 -L June 6th afforded an unusually favourable opportunity 

 for observations, both with and without the aid of the camera, 

 on the character of lightning-flashes, and for determining 

 the causes of some of the curious effects noticed by previous 

 observers. 



While watching the storm from my house in Ealing I 

 could in several instances distinctly perceive a flickering 

 appearance in a discharge, and in one particular case the repe- 

 titions were at least 5 or 6 in number, just sufficiently slow 

 for the eye to detect the variations in brightness without re- 

 moving the impression of one single flash. Other observers 



* Communicated by the Physical Society : read June 8, 1889. 



