14 
Professor Leslie on Electrical Theories. 
electrified particles of air which are immediately adjacent to A 
and B, will be first attracted, and then repelled on both sides 
towards the middle C, where these contrary motions, assisted by 
the repulsions from the points A and B, will produce an accu- 
mulation. And, thus the increased elasticity of the particles at 
C will obstruct, or totally prevent, the accession of the external 
air, and the electrical stream will cease. When the two points 
are held parallel to each other, as in Fig. 4., the repelled streams 
will meet at C, some distance from the middle, and as this con- 
cussion will cause an augmentation of density, the afflux of the 
air will be somewhat checked. But the directors of the currents 
from A to C, and from B to C, are very oblique, and, therefore, 
though a part of their force be spent in occasioning a compres- 
sion at C, a greater part will be employed in making the par- 
ticles recede from the middle C, and consequently will admit a 
renewal of the stream. Accordingly, let two tapered wires be 
held parallel, and about a quarter of an inch from each other, as in 
Fig. 4., at the distance of half a foot or a foot from the prime 
conductor; and the points of both will be lucid, though rather 
fainter than if kept asunder. But if, while the points are in this 
situation, the wires be gradually opened till they are in opposite 
directions, as in Fi g . a, the light will be found to decay, and 
at last vanish entirely. 
J 
If the medium is denser, its motions will be more obstructed, 
and consequently the effect of the point diminished. Hence 
the improvement made on the electrometer, by including it in a 
small phial; for not only is it less disturbed, but the play being 
considerably checked, it acquires stronger electricity, and exhi- 
bits a more permanent repulsion. On the contrary, the motions 
will be performed with greater freedom in a rare medium, and 
the influence of the point will be augmented. It is thus that, 
in the imperfect vacuum of an air-pump, the electrical attrac- 
tions and repulsions can hardly be perceived, or rather they are 
quickly enfeebled. Hence also the reason of the singular fact 
observed by several travellers, that the electrical machine works 
badly on the summits of lofty mountains. There is a limit, how- 
ever, to this effect of the air’s rarefaction : For, though the 
stream has its celerity and extent thus vastly increased, it also 
becomes proportionally attenuated, and, in the end, a smaller 
