244 SCIENCE. 
paratus and the results obtained come 
largely from the apparent ability to secure 
a representation of the effects of lightning 
discharges upon a moderate scale, and the 
possibility of studying the action of air and 
other gases, as well as liquids and solids, 
at varying temperatures and pressures un- 
der high electric stresses. Broadly consid- 
ered, however, the similarity of the effects 
to those produced in a thunder cloud is 
more apparent than real. The globules of 
of water constituting the electrified cloud 
do not possess charges of millions of volts 
potential, the effects of which are seen in 
the stroke of lightning. The individual 
globules may possess only a moderate 
charge. When, however, they are massed 
together in a large extent of cloud the vir- 
tual potential of the cloud as a whole, with 
respect to the earth, may be enormous, 
though no part of the cloud possesses it. 
The cloud mass not being a conductor, its 
charge cannot reside upon its outer surface 
or upon its lower surface nearest the earth, 
as with a large insulated conductor. The 
charge, in fact, exists throughout the mass, 
each globule of water suspended in the air 
having its small effect. upon the total re- 
sult. 
When the cloud discharges, the main 
spark branches within and through the 
cloud mass in many directions. The dis- 
charge can at best be only a very partial 
one, from the nature of the case. These 
are conditions which are certainly not rep- 
resented in our experimental production of 
high-potential phenomenon, except perhaps 
upon a very small scale in the electrified 
steam from Armstrong’s hydroelectric ma- 
chine, a type of apparatus now almost obso- 
lete. Yetif we wish to reproduce, as nearly 
as possible upon a small scale, the condi- 
tions of the thunder cloud, we shall be 
compelled to again resort to it.. In volcanic 
eruptions similar actions doubtless occur 
and give rise to the thunder clouds which 
[N. 8. Vou. X. No. 243. 
often surround the gases sent out from the 
crater. 
Considering, then, thatthe conditions in 
the thunder cloud are so different from those 
in our experiments with high potentials, we 
can easily understand that the study of light- 
ning phenomena may present problems diffi- 
cult tosolve. Two forms at least of lightning 
discharge are quite unknown in the labora- 
tory—namely, globular lightning and bead 
lightning, the latter the more rare of the 
two. Personally I cannot doubt the exist- 
ence of both of these rare forms of electri¢ 
discharge, having received detailed accounts. 
from eye witnesses. Onone occasion, while 
observing a thunder storm, I narrowly 
missed seeing the phenomenon of globular 
lightning, though a friend who was present, 
looking in the opposite direction, saw it. 
The explosion, however, was heard, and it 
consisted of a single detonation like the 
firing ofa cannon. According to the testi- 
mony of an intelligent eye-witness, who de- 
scribed the rare phenomenon of bead light- 
ning within an hour after it had been seen, 
itis a very beautiful luminous appearance 
like a string of beads hung in a cloud, the 
beads being somewhat elliptical and the 
ends of their axes in the line of their dis- 
charge being colored red and purple re- 
spectively. This peculiar appearance, not 
at any time dazzlingly bright, persisted for 
a few seconds while fading gradually. 
Again, our knowledge of the aurora is 
not as yet much more definite or precise 
than it is in regard to the obscure forms of 
lightning alluded to above. Whether these 
phenomena will ever be brought within the 
field of research by experimental methods 
is an open question. 
The endeavor in the foregoing rather dis- 
connected statements has been to indicate 
directions in which the field of experiment 
may be extended and to emphasize the fact. 
that research must be carried on by exten- 
sion of limits, necessitating more liberal en- 
