338 



Measured from the right-hand point to the striking spot on the left- 

 hand disk, or another point nsed in place of it, the theoretic lengths of 

 these sparks might be from fifteen to seventeen inches ; but if we 

 considered the twists and differences of direction of their several zig- 

 zags, their real length in every case was much more ; and in some 

 instances it must have been, at least, twice as great as the distance from 

 the point to the spot struck on the disk. 



In several instances the long sparks appeared to the eye to form 

 loops, but this was evidently due to their adopting a somewhat spiral 

 form. This peculiarity of form has been also noticed in lightning. As 

 equivalents of flashes of real lightning, these long sparks should possess 

 great interest to electricians. 



Though their motion in space appeared to us to be due to blind 

 chance, yet that notion cannot be adopted by physicists, who must 

 work out reasons for the whip-lash appearance of these sparks, instead 

 of the taut cord or right line direction of other electric sparks. The 

 long forked sparks produced by frictional electricity differ materially in 

 their form and colour from those produced by the induced helix. The 

 two kinds of sparks should be compared together at the same time, and 

 as much as possible under similar circumstances. 



'No doubt the application of photography to real lightning on the 

 great scale, and to these long induced electric sparks on the small scale^ 

 may lead us to the exact knowledge of their likeness or unlil^eness in 

 form, which the human eye cannot perceive. This application may have 

 been made already ; but, if it has, I am not aware of the fact. The sug- 

 gestion will occur to any one who takes the same view of this subject 

 with the author. 



Hitherto the freaks of flashes of lightning in apparently avoiding 

 conducting rods, and iron chimneys of steamers, and in striking objects 

 near them, whether composed of good or bad conducting material, are 

 facts which throw a great doubt on the advisability of using metallic 

 conducting rods to buildings and ships. Theory in these cases is at 

 fault : something remains to be worked out, to account for apparent 

 exceptions to the law of ''least distance;" and as these sparks appear 

 to be flashes of lightning on a small scale, and perfectly manageable 

 by the experimental philosopher, I notice them here in the hope that 

 the law of their forms and directions may be studied hj parties who 

 have the means at their command for thoroughly sifting and tracing the 

 causes of the phenomena noticed in this communication. 



It was observed by Mr. Yeates, who was present at the lecture, that 

 though there is a wonderful likeness in the forms of the long sparks 

 produced by the induction coil and zigzag flashes of lightning, they 

 were not accompanied with the smell of ozone, which is common to 

 lightning and machine electric sparks ; and that, consequently, there 

 may be a real difference between the induced electric discharges and 

 those which accompany ordinary electric phenomena. Indeed, theory 

 would lead to the conclusion that these induced sparks are double, an 



