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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 neoative. 



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, fcc, 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 tliis 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 tliei'e 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 



