334 PHYSICAL GEOGRAPHY. 



to notice the incontestable increase of the tension of positive 

 electricity accompanying the increased elevation of the 

 station and the absence of neighbouring trees, ( 413 ) its 

 daily variations, which take place, according to Clarke's 

 experiments at Dublin, in more complicated periods than 

 those found by Saussure and myself, and its variations at. 

 different seasons of the year, at different distances from the 

 equator, and under different relations of continental or 

 oceanic surface. 



The electric equilibrium is more rarely disturbed where 

 the aerial ocean rests on a liquid base, than where it rests 

 on land ; but it is very striking to notice in extensive seas 

 the influence which small groups of islands exercise on the 

 state of the atmosphere in occasioning the formation of storai9. 

 In fogs, and in the commencement of falls of snow, I have 

 seen the previously permanent vitreous electricity pass rapidry 

 into resinous, and the two alternate repeatedly, during long 

 series of experiments made both in the plains of the colder lati- 

 tudes and in the paramos of the Cordilleras, at elevations from 

 11000 to 15000 English feet under the tropics. The alter- 

 nate transition was quite similar to that shewn by the electro- 

 meter a short time before and during a thunderstorm. ( 414 ) 

 "When the vesicular vapour has condensed into clouds with 

 definite outlines, the electricity of the separate vesicles of 

 vapour passes to the surface of the cloud, and contributes to ilk- 

 crease the general electric tension of the exterior surface. ( 415 ) 

 According to Peltier's experiments at Paris, slate-grey clouds 

 have resinous, and white, rose, and orange-coloured clouds 

 have vitreous electricity. Thunder clouds not only envelop 

 the highest summits of the Andes, (I have myself seen the 

 vitrifying effects of lightning on one of the rocky pinnacles 



