636 , ALEXANDER MACFARLANE ON THE 
got by multiplying the former function of 7 by °642 + = satisfies all the — 
observed values up to 22°5 centimetres ; and in the case of the second curve, — 
the equation 
V = 6081-77 —* — 42:262 
satisfies all the values of 7 beyond 24 centimetres ; while the equation 
V = 3188-47 + 98:32 — 839707, 
got by multiplying the former function of 7 by 52427 + :0198227, satisfies all 
the preceding observed values. The figures show these peculiarities well. As 
the curve is not discontinuous, the true function of 7 for V must coincide with 
the former function when 7 is large, and with the latter when 7 is small. The 
observations of 20th July, where a single observation was taken for each value 
of 7, may be taken as showing the degree of precision finally arrived at in the 
experiments. 
In the sketch there is represented on the corner of the table a long range 
absolute electrometer, which the Professor of Natural Philosophy procured on 
loan for me from Sir W1Li1AM TuHomsov, for the purpose of reducing the results — 
to absolute measure. To provide for this reduction, I took each day four or 
five observations of the reading given when a spark passed between the two — 
parallel metal discs at a distance of ‘5 centimetre, the insulating air being at 
the pressure of the atmosphere. I compared the electrometers on five days by 
the following method :—I connected the insulated disc of the absolute electro- 
meter with the insulated ball A, thus substituting the disc wire and ball for the 
second system of conductors. A charge was communicated to the substituted 
system sufficient to attract upwards the aluminium square of the balance ; and 
the reading of the wire-image watched until from a.slow escape of the charge 
along the insulating stems of the absolute electrometer, the attraction was just 
insufficient to keep the square in its sighted position. This method was found 
to give more consistent readings than when the insulating stems of the absolute 
electrometer were dried, and a coincidence obtained by moving the guard plate 
up or down. The observations on one of the days are given in Table XXVIII. 
The equation, by means of which the coefficient was deduced from the given 
data, is ta + 
V/—V=(D’—D) VE. 
The mean of eight independent determinations shows the potential required — 
to pass a spark between the discs when at a distance of ‘5 centimetre, and 
separated by air at atmospheric pressure to be 38°63 C.G.S. units. 
The method above described has enabled me to investigate several of the 
laws of the disruptive discharge of electricity. 
[Added January 1878.—In the tables appended we have entered almost all 

