On Electricity. 263° 
at ‘some distance from each other, and sometimes consider- 
ably so, being at the opposite extremities of the slip: but 
each impression was immediately opposite to the perforation 
in the paper contiguous to it; soahat an upward impression, 
and an impression downwards on two opposite tinfoil slips, ° 
resulted from the perforation of each interposed stratum of 
paper: which is a sufficient proof that the effect arises from 
the expunsion caused by a spark at each interruption of the 
metallic circuit, and not from any effect of opposite cur- 
rents. 
When a card or quire of paper is pierced by a charge 
passed through wires applied to its opposite surfaces, there 
is but.one interruption of the metallic circuit, and conse- 
quently but one spark; this produces an expansive effect, 
which willoperate with ihe greatest force where it finds 
the least resistance; which will be at the opposite surfaces 
of the card.or paper: consequently a bur or protrusion will 
take place at those surfaces, and if any soft substances are 
contiguous, they will receive an impression, although they 
are out of the circuit, and of course could not be affected by 
a current either way. But if a series of cards or strata of 
paper are placed on each other, and separated by as many 
slips of tinfoil, the interruptions in the metallic circuit will 
be as numerous as these slips; and as a spark will take 
place at each interruption when a charge is passed through 
them, an expansive effect will be produced by the perfora- 
tion of each stratum of paper, and this expansion must 
necessarily indent the tinfoil contiguous to its opposite sur- 
faces, and consequently produce two indentations in op- 
posite directions. The slips of tinfoil may be considered 
as stepping-blocks for the electric fluid, which does not (in 
consequence of their extension) pass in a right line, but 
through the least resisting part of each interposed stratum 
of paper. 
The truth of this explanation is evident from other phe- 
nomena attending the experiment ; for, allowing it to be 
correct, the effect of expansion may be expected to be most 
considerable when the slips of tinfoil are at the greatest di- 
stance from each other, as the spark then passes through 
a greater interval; and it will be found that the impressions 
are better defined and deeper, when several sheets of paper 
are interposed between each pair of tinfoil slips, than when 
they are only separated by a single sheet. 
An additional confirmation of this fact has also occa- 
sionally occurred in my experiments, In varying Mr, 
R4 Symmer’s 
