of the two electricities. Sometimes the discharge is only from cloud to
cloud, sometimes from the earth to the clouds, and sometimes from the
clouds to the earth, as one or other may be in the positive or negative
state. The clouds are usually more or less electrical when the vapour,
floating about in the atmosphere, is condensed, and the earth being
brought into an opposite state of electricity by induction, a discharge
takes place, when the clouds approach within a certain distance, and
sometimes the electric cloud perches upon a hill, and then discharges
itself. The electricity passes through the clouds in a zig-zag
direction, and the undulation of the air which it produces is the cause
of the noise which we hear, called thunder, which is more or less
intense, and of longer or shorter duration, according to the quantity of
air acted upon, and the distance of the place where the report is heard
from the point of discharge. If the danger be great, we have seldom any
opportunity to count the time which elapses between the appearance of
the lightning and the report: electrical effects take place at no
sensible time; it has been found, that a discharge through a circuit of
four miles is instantaneous, whilst sound moves at the rate of about
twelve miles in a minute. So that, supposing the lightning to pass
through a space of some miles, the explosion will be first heard from
the point of the air agitated nearest to the spectator; it will
gradually come from the more remote parts of the course of the
electricity, and, last of all, will be heard from the very extremity;
and the different degrees of the agitation of the air, and the
difference of the distance, will account for the different intensities
of the sound, and its apparent reverberations and changes. If you can
count from two to three seconds between the appearance of the lightning
and the sound, there is seldom much danger; and when the interval is a
quarter of a minute, you are secure.--_Brande's Lectures.--Lancet._