3\0 DESCRIPTION OF MOUNT MEZIN. 



Condu.trs for certain condticting parts lying along or within the earth 

 lightning. either as ridges or internal masses, and that the stroke from 

 a stratum of clouds, many mileb in length, seems to be de- 

 termined by an nction which extends far beyond the influ- 

 ence of iny metallic rod, — even supposing this last to be in- 

 serted into the conducting mass itself: that the whole pro- 

 cess of atmospheric evaporation and condensation appear to 

 be accompanied with electric phenomena upon a very ex" 

 tended scale, but most strikingly manifest when the changes 

 are rapid ; this last being the only difference between thun- 

 der storms and common squalls or showers : and that it does 

 not seem probable, that our rods can essentially modify the 

 conr&e of these effects. Other more remote considerations 

 would offer, if they could ^ such as the possibility of an in- 

 terruption of the ordinary course and frequency of showers, 

 ' which Darwm thought within the reach of human power, 

 and the greater probability that the atmospheric electricity 

 of a whole country would soon destroy a>iy series of conduc- 

 tors: but the affair of the poor-house at Heckingham *, in 

 Norfolk, which, about thirty years ago, was struck and set 

 on fire by lightning without touching any one of eight ele- 

 vated metallic conductors attached to the building, has been 

 considered as a proof of the very limited influence of these 

 Tods, and tha*^^ their power of proftecting a single edifice re^ 

 quires the condition, that all the conductors should be con- 

 nected together, and with the metallic parts of the house. 



XV. 



Extract of a Letter from Mr. Cordier, Mine Engineer, on 

 Mount Mezin f. 



Mount Cenls. jl_ HE passage of Mount Cenis has been laid open to view 



Alternation of by the new road. We see there vast strata of gypsum, which 



gypsuni and alternate with the rocks of micaceous schist, compose nearly 

 micaceous . . , 



schist, a twentieth of the mass of the mountains, and show thcm- 



* See Philos. Trans, of that time. 



t Journal des Mines, Vol. XXVI, p. 230. 



selves 



