the Discharge of a Leyden Jar. 161 
locity ; and as several discharges occurred in a second, it was 
seldom that the weight ran down without at least one good 
observation being obtained. 
ith this mirror making not less than 800 turns in a second, 
I was greatly surprised to find that the image of the spark on 
the ground glass, as viewed by the naked eye, was quite un- 
aifected in appearance, looking about the same as though the 
muror had been stationary. This experiment, which was re- 
daily, gave uniformly the same result, and proved that 
the total duration of the discharge was incomparably shorter 
in the case of the larger jar. When the paper with the 
black lines ruled on it was used, they were seen equally distinct 
with the highest as well as with the lowest velocities; and all 
the evidence went to show that the discharge of this small jar 
consisted of a single act, whose duration was immeasurably short. 
Knowing well the inestimable value in certain physical inqui- 
nes of a source of illumination of this character, a series of mo 
deliberate experiments were now instituted for the purpose of 
examining in detail its nature, and, if possible, duration. 
_ The mirror was made to revolve 300 times in a second, the 
mage received on the ground glass, and viewed with the naked 
eye, platinum wires ,', of an inch in diameter being used, with 
a striking distance of five millimeters. The result was as above 
given, the spark-image being totally unaffected—once only at 
one of its ends a very small and faint streak was noticed. Re- 
is Previously employed. The platinum wires were replaced 
+ ot 
assum by th p } s 
though for practical purposes the duration of this faint gs or 
of iny : ve the mi- 
int to be gained was 
Someter always eR. to the object to be measured; the 
‘Park. must carry its own micrometer—must In some way 
“<-Image 
be made as it were to measure itself. 
