"The lightning flash itself never lasts more
than 1/100000 of a second." It is, however, just as likely that a
discharge may travel upward as downward. What controls the discharge? Does
the quality of the charge?--that is to say, is the positive or the
negative more prone to break disruptively through the insulating medium?
Investigations with Geissler's and other tubes containing highly rarefied
gases have made it tolerably clear that there is a greater "tearing away"
influence at the negative than at the positive pole, and if two equal
balls, containing one a positive and the other a negative charge, be
equally heated, the negative is more readily dissipated than the positive.
But, so far as we at present know, this question enters into the
discussion scarcely, if at all. Our knowledge seems rather to point to the
substances upon which the charges are collected. The self-repellent nature
of electricity compels it to manifest itself at the more prominent parts
of the surface, the level being forsaken for the point. The tension of the
charge, or its tendency to fly off, is proportionately increased. And if
at a given moment the tension attains a certain intensity, the discharge
follows, emanating from the surface which offers the greatest facilities
for escape. The earth is generally flatter than the cloud, whence, in all
probability, the discharge more frequently originates with the cloud.

Should a lightning flash strike the earth and produce direct neutrality,
it is possible that no damage will result, although this again is not
always certain, because when the cloud charge acts inductively on the
earth it produces the opposite (say negative) charge on the nearer parts,
the similar (or positive) state is also produced at some place more or
less distant. Sometimes this "freed" positive (which, by the way,
accumulates gradually and physiologically imperceptibly) is collected at
some portion of the earth's surface. When the negative is neutralized by
the discharge, the freed positive is no longer confined to a particular
region, but tends to dissipate itself, and a shock may be felt more or
less severely by any person within the region. Or, again, a similar shock
may be experienced by a person standing within the negative zone on the
neutralization of the charge.

I may take the opportunity here to mention a highly interesting and
instructive incident observed on local telegraph circuits during a
thunderstorm. The stor