162 ANNUAL OF SCIENTIFIC DISCOVERY. 



the sectors on one side ; from this portion I removed every con- 

 ductor as far as possible ; still, the machine in action is surrounded 

 by such an atmosphere of electricity, that there is danger of being 

 deceived in the results sought. I took proof planes of 2 and 

 3 inches in diameter, with lac handles, to take the charge and 

 bring to the gold leaf electrometer, stopping the plate suddenly 

 and applying the proof planes, sometimes to both sides, and 

 sometimes to one side, simultaneously or alternately. They, in 

 general, indicated opposite electricities and different intensity in 

 opposite sides of the plate ; yet, in a large number of trials, made 

 as nearly as possible in the same place relatively to the sectors, 

 the results were so contradictory that nothing could be deter- 

 mined, sometimes the one side was positive, sometimes neg- 

 ative. 



"I then applied the proof plane on one side and a disc of metal 

 of larger diameter to the opposite side in connection with the 

 earth, thus leaving the electricity on the side of the proof plane 

 free, and by repeating many times the experiment in the same 

 place, on both sides, I was surprised to find both sides of the re- 

 volving plate of the same kind in all cases ; negative when the 

 preceding sector had been excited positive. The two sides of the 

 sectors, when removed, show also the same kind. 



" The role of the papers upon the sectors is far from clear; they 

 may be on either or both sides of the glass. Of a number, made 

 as nearly alike as possible, a few may act well ; the varnish of 

 some may be scratched even through the paper in all directions, 

 and with slight loss of effect. Others may resist all excitation, 

 but, by doubling a piece of tin foil over the edge, may perform 

 well. The paper may be left off entirely, and a piece of tin foil 

 doubled over the edge, as large on each side as the paper is 

 usually made, retaining a card-point feeder, without varnish, and 

 yet this may do very well for at least one of the sectors, where 

 only a pair is used. 



" The power of the instrument for evolving ozone is very great, 

 and I have reason to caution those using the machine in a confined 

 atmosphere." 



According to " Comptes Rendus," Feb. 11,1867, Bertsch's ma- 

 chine is the same as that of M. A. Piche, except that the former 

 substitutes " hardened rubber " for " paper covered with several 

 layers of gum lac." 



For a full description of the Holtz machine, the reader is re- 

 ferred to the " Journal of the Franklin Institute," Oct., 18G7. 



LIG HTNING-COND UCTORS . 



Tn a report addressed by M. Pouillet to the French Minister of 

 War, in the name of a commission appointed to examine the best 

 way of securing powder magazines from the effects of lightning, 

 we find the following interesting remarks : 



" When, dur.ing a thunder-storm, a cloud charged with a given 

 kind of electricity approaches the earth, the latter becomes at 

 that spot charged with the contrary kind of electricity. This, 



