a Thunder-storm as observed at Woolwich. 419 



carried over Woolwich in the direction of the wind, the most 

 formidable group of them, and consequently the greatest fury 

 of the storm, were deflected out of the wind's track before 

 their arrival at Shooter's Hill, and were carried over the low 

 lands on the other side of the hill, towards the Thames, in a 

 direction nearly from W.S.W. to E.N.E. 



The deflection of electrized clouds out of the wind's direc- 

 tion, though, perhaps, not much noticed, is a very common 

 circumstance in the neighbourhood of high lands, especially 

 if those lands are composed of materials which are bad con- 

 ductors of electricity. For although they do not absolutely 

 refuse the transmission of the electric matter driven to the 

 surface of the hill from the lower strata of air* by the dis- 

 turbing force of the condensed electric fluid in the clouds, 

 the transfusion into the ground is too tardily performed to 

 prevent accumulation on the surface, which consequently be- 

 comes charged in the same state as the clouds that are ap- 

 proaching it. A reaction immediately takes place, and a con- 



* The asperifolious plants, and the vegetable clothing of the land ge- 

 nerally, especially at this season of the year, receive the electric fluid from 

 the atmosphere in great abundance ; and the myriads of vegetable points, 

 sharp edges, &c, presented to the air, offer every facility for its reception 

 on any emergency of pressure emanating from the repulsive force of a 

 highly charged cloud. The surface of the land thus becomes charged at 

 the expense of the air, each gradually resuming its natural electric equili- 

 brium again, as the disturbing force withdraws its influence, by the pro- 

 gress of the cloud in its course. Thus new tracts of country become 

 charged in succession as the cloud approaches them, and an electrical tide 

 sweeps the face of the land by the floating influence above. 



It is on this account that insulated kite-strings, exploring rods, &c, fre- 

 quently become negatively electric at the approach, and during the transit, 

 of clouds of this description. But if the kite or the exploring rod were 

 to reach into the cloud, it is not likely that either of them would ever be 

 found in a negative state. I am speaking of the principal influencing 

 cloud, and not of those straggling thin patches in their vicinity, which fre- 

 quently became negative by a portion of the electric matter which they 

 before possessed being driven out of them by the predominating electric 

 force of the superior cloud. In the same manner, an insulated metallic 

 rod furnished with fine points or sharp edges at its further extremity, may 

 have its natural electric fluid driven out of it into the air, by the approxi- 

 mation of a positively charged body at the other end. 



I have never yet found the atmosphere negative with regard to the 

 earth at any other time than when modified by such causes as I have 

 pointed out. I have made upwards of four hundred electric kite experi- 

 ments, under almost every circumstance of weather, at various times of 

 the day and night, and in every season of the year; I have experimented 

 on Shooter's Hill, and on the low lands on the Woolwich and Welling 

 sides of it, and the experiments in the three different places within an 

 hour of each other; I have done the same on Chatham lines, and.in the 

 valley on the Chatham side of them ; on Norwood Hill, and in the plain at 

 Addiscombe; also on the top of the Monument in London, and during 



3 H2 



