642 ALEXANDER MACFARLANE ON THE 
amount of electrification introduced inside the electrometer. Between the fifth — 
and sixth observations the apparatus was disturbed, which accounts for the 
larger differences. The observations of this table, taken together with those of 
Table XX., show that the curves for the continued discharge are similar to 
those for the single, but are of smaller magnitude. 
Measurement of the Difference of Potential required to produce the same length 
of Spark at different pressures of the Dielectric. 
In this independent variation we have not the complexity introduced 
by a change of distance between the electrodes. We accordingly have 
simpler equations; the simple hyperbola satisfies well all the points of 
observation. 
The observations of Tables XVII. and XVIII. were taken in immediate 
succession. The latter give very accurately an hyperbola, the former require a 
small correction. I think that the latter curve was taken under more favour- 
able auspices as regards the state of the observers ; the readings in themselves 
were more difficult to take, on account of the slow velocity with which it was 
necessary to drive the machine owing to the small capacity of the conductor. 
The observations of Tables XIX. and XX. also give hyperbolas. The results” 
are,— 



Table. Function for V. a. b. 
XVIL 04798 ./{p®+205°58p} 102°79 46678 
XVII. | 044551 /{p?+200p} 100 44551 
XIX. 046342 / (tp? +199-01p} 99°51 4°6112 
XX ‘046853 ,/{p?+ 207-06p} 103°53 4°8508 
Mean 045794 ./ {p®-+202-92p} 101-46 46462 
where a and & denote the semi-axes of the hyperbola, and p the pressure in 
millimetres of mercury. 2 for XVIII, which is the curve obtained without 
Leyden jars, is smaller than for any other of the curves. 
The observations for the contmued discharge, Table XX., were taken each 
after the reading for the corresponding single discharge. By neglecting the 
higher terms of p, we get 
V = 035032 / {p+ 205'62p} , 

