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us — we know that lightning is electricity and that is all. 
It is not, I think, decided whether the storm is incidental 
on the electrical disturbance or vice versa, i.e., whether the 
electricity causes the clouds and storm or is a mere attendant 
on them. Nor can I ascertain that there is any certain infor- 
mation as to whether, when the discharge is between the 
earth and the clouds, the clouds are positive and the earth 
negative, or vice versa. Such information as I can get 
appears to point out the following law : that in the case of 
a fresh -formed storm, the cloud is negative and the earth 
positive ; whereas, in other cases, the cloud is positive and 
the earth negative. 
Again, thunder storms move without wind or indepen- 
dently of wind ; but I am not aware whether any law con- 
necting this motion with the time of day, &c., has ever been 
observed, though it seems natural that however complicated 
by wind and other circumstance, some such law must exist. 
In this state of ignorance of what the phenomena of thunder 
really are it is no good attempting to explain them. What 
I shall do, therefore, is to shew how the inductive action 
of the Sun would necessarily cause certain clouds to be 
thunder clouds in a manner closely resembling, and for all 
we know identical with, actual thunder storms. 
In doing this I assume that the thunder is only an 
attendant on the storm and not the cause of it • 
and that many of the phenomena such as forked and 
sheet lightning are the result of different states of 
dampness of the air and different densities in the 
clouds, and really indicate nothing as to the cause of 
electricity. In the same way, the periodicity of the storms 
is referred to the periodical recurrence of certain states of 
dryness in the atmosphere. Thus the fact that there is no 
thunder in winter is assumed to be owing to the dampness 
of the air which allows the electricity to pass from and to 
the clouds quietly. What I wish to do is to explain the 
