SIR JOHN HERSCHEL. 



the time of Bacon, philosophers have discovered, 

 that electricity can produce all the above effects 

 without being a material fluid others, that it 

 proceeds from the earth to the clouds, and not 

 from atmospheric vapour to the earth while 

 others again resolve all its mechanical and heat- 

 ing properties into undulations of the " unknown 

 aether." 



When I come to treat of Voltaic phenomena, 

 it will be seen that the power of electricity to 

 produce a shock, and the velocity with which it 

 passes through conductors, are modified by the 

 various modes of its disengagement from ponder- 

 able matter. 



With his well known sagacity, Sir John Her- 

 schel has laid down some admirable rules for 

 the purpose of facilitating our research, among a 

 great mass of assembled facts, for their common 

 cause. They are substantially the following : 



1 . Invariable connexion of the cause and effect, 

 unless prevented by some counteracting cause. 



2. Invariable negation of the effect, with ab- 

 sence of the cause, unless some other cause be 

 capable of producing the same effect. 



3. Increase or diminution of the effect, with 

 the increased or diminished intensity of the 

 cause in cases which admit of increase and di- 

 minution. 



4. Proportionality of the effect to its cause in 

 all cases of direct unimpeded action. 



