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XXV. On the Experiment of Mahomet's Coffin. 

 By G. Johnstone Stoney, M.A., F.R.S. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, Dublin, August 12, 1868. 



MR. W. F. BARRETT, during a visit to Dublin last spring, 

 showed an uncommonly beautiful electrical experiment, 

 which I understood him to say was described in the older trea- 

 tises on electricity, but appeared to have since dropped out of 

 recollection. I do not find any reference to it in such books as 

 I have access to. You may perhaps, then, not deem it a waste 

 of your space to recall attention to it, and insert an explanation 

 of a phenomenon which at first sight seems very marvellous. 



The experiment is easily made. Raise the knob of a Leyden 

 jar to a considerably greater height than is usual. This may be 

 done by boring a hole in the top of the ordinary knob of the jar 

 and inserting into it a stem 3 or 4 decimetres long, carrying an- 

 other knob. Charge the jar, and then drop down a little strip 

 of gold-leaf towards the knob from a height of a couple of deci- 

 metres. The strip may be from 3 to 5 centims. long, and an 

 attempt should be made to cut it into a form somewhat sharper 

 at one end than the other. This is best done by heating a sharp 

 penknife so as thoroughly to dry it, and then cutting the gold- 

 leaf with tolerable pressure, and by a motion resembling minute 

 strokes of a saw. All that need be aimed at is that the fragment 

 have either more or sharper jags at one end than the other. If, 

 when it is dropped towards the knob, the sharper end happens to 

 be directed downward, the strip of gold will touch the knob and 

 immediately dart away again ; but if it be dropped with the 

 blunter end downwards, it will approach without touching the 

 knob and take up a position in its vi- 

 cinity apparently without any support. 

 There it will continue in mid air so 

 long as the jar remains sufficiently 

 charged. 



The explanation may be conveniently 

 divided into two parts — one respecting 

 the distance at which the fragment of 

 gold-leaf will establish itself, and the 

 other explaining how its weight is sup- 

 ported. 



At all distances the strip is charged 

 by induction, electricity unlike that of 

 the jar (which we shall suppose positively 

 charged) being driven to the further 

 and sharper end, while like electricity 



