-1859] WRITINGS OP JOSEPH HENRY. 335 



dition observed and another needle put in its place, before 

 the noise of the thunder reached the ear. In this experi- 

 ment the inductive action of the electrical discharge in the 

 heavens was exerted on the natural electricity of the tinned 

 roof, (a surface of 1,600 square feet,) and a considerable portion 

 of this passed down through the wire into the well. The 

 arrangement served to indicate an action which would 

 otherwise have been too feeble to produce a sensible impres- 

 sion. 



It must be observed that the effect here described was not 

 produced by the actual transfer of any electricity from the 

 cloud, but was simply the result of induction at a distance 

 and would probably have been nearly the same had the in- 

 tervening space been filled with glass or any other solid 

 non-conducting substance. We say probably very nearly the 

 same, because Professor Faraday has shown that the induc- 

 tive effect at a distance is modified by a change in the in- 

 tervening medium. 



It is also proper to mention here, (although we cannot 

 stop to give the full explanation of the means by which 

 the result was obtained,) that the electricity passing along the 

 wire was not that due to a single discharge into tlie well, 

 but to a series of oscillations up and down in alternate direc- 

 tions until the equilibrium was restored. 



Electricity in Motion. — The phenomena we have thus far 

 described relate principally to electricity at rest. Those 

 which relate to ordinary or frictional electricity in motion 

 have not been so minutely investigated as the other class, 

 and present much more difficulty in ascertaining the laws 

 to which they are subjected. The discharge of electricity 

 from the clouds or from an ordinary electrical machine is 

 so instantaneous that we are principally confined in our in- 

 vestigations to the effects which remain along its path after 

 its transfer. 



The electricity however which is developed by chemical 

 action in a galvanic battery is of sufficient quantity to pro- 

 duce a continuous stream, or at least a series of impulses in 

 such rapid succession that the}'' may be considered continuous. 

 By employing electricity of this kind, it has been supposed 



