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One of the brafs plates, which are foldered to 
the ferules, hath a fmall hole drilled through 
it, by which the air is to be exhaufted. It 
is covered by a ftrong valve, properly fe- 
cured, and concealed by the brafs ball B,orC. 
D.E. Balls of brafs, about five eighths of 
an inch in diameter, fixed upon wires, 
which projeCt two inches and an half 
from, the brafs plates, at each end of the 
glafs-tube. 
F. A fine-pointed wire, to colleCt the elec- 
tricity from the excited glafs-globe, &c. 
G. Supporters, of fealing-wax ; upon which 
the luminous conductor is to be mounted. 
N. B. The dots in the tube are intended to re- 
prefent the appearance of the electricity in 
it, in the experiments defcribed in Tab. xiv. 
But, when a bottle, or a large jar, is difcharged 
through the glafs-conduCtor, it is uniformly 
filled with light. 
The use of the glass conductor. 
The glafs-tube, thus furnifhed, and mounted, 
being properly exhaujled, and perfectly dry, will aft 
in all refpeCts like one of metal ; and the electro- 
meter, being placed upon the brafs ball B. will 
anfwer to the charge of a jar, or battery, exaCtly. 
But the principal ufe of this inftrumenr, is to af- 
certain the direction of the eleElric matter , as it pajfes 
through it. And this end, it completely aniwers 
in the manner following, viz. fet it with the col- 
leCting-point F. before the globe, and place the 
knob 
71 
