DE. EYEEETT OjY ATMOSPHEEIC ELECTEICITY. 
351 
insulated half-ring, according as the potential of the conductor connected with this half- 
ring differs positively or negatively from that of the other conductor (earth) connected 
with the case. The mirror turns accordingly in one direction or the other through a 
small angle from its zero position, and produces a corresponding motion in the image of 
the lamp on the screen on which it is thrown. 
In the Kew apparatus, this image was thrown upon photographic paper, which was 
drawn upwards with a uniform motion by clockwork, and a continuous trace of the 
variations of electrical potential was thus produced. A zero line was at the same time 
drawn by the image of the same flame reflected from a fixed mirror. 
The curves of atmospheric electricity thus obtained are about 18 inches long; and 
each sheet contains two. Each curve embraces a period of about twenty-four hours, the 
paper having been regularly shifted or changed at about half-past 10 a.m. 
Generally speaking, the curves are distinctly traceable through the whole twenty-four 
hours. The interruptions which do occur are owing, in some cases, to the spot of light 
having moved too fast to leave a trace, in others to its having passed off the paper. As 
regards this latter source of failure, it may be remarked that it is not detrimental to the 
investigation of the law of diurnal variation. It merely does for us what General Sabine 
found it necessary to do in combining magnetic observations, that is to say, it rejects 
observations at times of unusual disturbance. 
Specimens of the curves, of the actual size, are given in Plate XIX. 
The ordinates (positive or negative) of the curves are to a close degree of approxi- 
mation proportional to the potential (positive or negative) of the air at the place of 
observation, provided that the charge of the Leyden phial (3) be preserved constant; 
and if this charge be allowed to vary, the ordinates will vary in simple proportion. 
The charge was tested daily by the gauge-electrometer, which we shall now describe, 
and which is identical with the station electrometer used in my own Windsor observa- 
tions. Its external appearance is shown in fig. 1, Plate XVIII., and some internal parts 
in figs. 2, 3, Plate XVIII. The same letters denote the same parts in all three figures. 
It consists of 
(1) A thin flint-glass bell (fig. 1, Plate XVIII.) coated outside and inside like a Leyden 
phial, with the exception of the bottom inside, which contains a little sulphuric acid (H). 
The dotted line A A indicates the boundary of the tinfoil. 
(2) A cylindrical metal case (shaded in fig. 1, Plate XVIII.), inclosing the glass jar, 
cemented to it round its mouth outside, extending upwards about 1-g- inch above the 
mouth, and downwards to a metal base supporting the whole instrument, and protecting 
the glass against the danger of breakage. 
(3) A cover of plate glass (C) with a metal rim, closing the top of the cylindrical case 
of the instrument. 
(4) A torsion-head (B, fig. 1, Plate XVIII) after the manner of Coulomb’s balance, 
supported in the centre of the glass cover, and bearing a glass fibre (E, figs. 1, 2, Plate 
XVIII.) which hangs down through an aperture in its centre. 
