382 PELTIER ON THE CAUSES OF THE 
12. A common electroscope (Wood-cut, fig. 1.) must be taken, — 
furnished with a stem not exceeding 40 centimeters, and ter- 
minated by a polished ball of metal from 7 to 8 centimeters in 
diameter. We shall presently say why we terminate the stem by — 
a large polished ball instead of terminating it by a point. This 
ball may be of glass or card, covered with tin foil in order that it 
may be lighter. If,—under a clear sky, and in a place which is 
quite open, commanding the neighbouring trees and buildings, in 
short, higher than all the surrounding bodies which rest upon 
the earth,—if in this position the stem and the plate of the in- 
strument are made to communicate in order to put them in an 
equality of reaction, it is then in equilibrium, and the gold leaves 
fall perpendicularly and mark zero. As this communication be- 
tween the stem and the plate may be established at all heights, — 
an electrometer may therefore be placed in equilibrium at any 
stratum and stand there at zero. Thus in equilibrium, it may 
be presented to the agitations of the atmosphere during entire 
hours without the leaves manifesting the least change in their 
equilibrium, if it be always kept at the same height*. This will 
no longer be the case if it be in the vicinity of a body projecting 
above the ground; this body would possess a tension so much — 
higher as it is more elevated and more pointed. It sometimes 
increases to so great a degree that the induced electricity ra-_ 
diates from its asperities under the form of a luminous brush, 
which phenomenon has been called St. Elmo’s fire. Horizontally 
receding from or approaching to a body like this gives the same 
result as vertically receding from or approaching to the ground. 
13. Instead of remaining in the stratum where the instrument 
has been rendered neutral, if the weather be dry, cold, and the 
sky perfectly serene, it is sufficient in our climate to raise it two 
decimeters in order to have 20 degrees of divergence with the 
gold leaves, but this divergence is much more considerable if 
the temperature has been for several weeks from 10 to 15 de- 
grees below zero; the elevation of a single decimeter is sufficient 
to project the gold leaves against the armatures. Beneath a) 
clear sky the electric sign is always vitreous. If during the 
day much vapour has been formed, it is necessary, in order to 
obtain the same intensity of action, to raise the instrument so 
much higher as the air contains more. Having obtained this 
* See the article ‘Atmosphere’ of the Supplement of the Dicf. des Se. Nat., 
and my communication to the Acad, des Sc. of the 8th February 1841. 
