ATMOSPHERIC ELECTRICITY 299 



unusual amount of cloud always forms a prominent feature in a great thunderstorm. 

 We have thus obtained one very important clue to the origin of the phenomenon. 

 In cases where the darkness is not so great, though the storm is visibly raging 

 overhead, it is always observed either that we are not far from the edge of its 

 area or, in rarer cases, that the lower cloud strata lie much higher than usual. 

 These form, therefore, no exception to the general conclusion just given as to the 

 abnormal amount of cloud present. Seen from a distance, the mass of cloud 

 belonging to the storm usually presents a most peculiar appearance, quite unlike any 

 other form of cloud. It seems to boil up, as it were, from below, and to extend 

 through miles of vertical height. The estimated height of its lower surface above the 

 ground varies within very wide limits. Saussure has seen it as much as three miles; 

 and in one case noticed by De 1'Isle it may have been as much as five miles. On 

 the other hand, at Pondicherry and Manilla it is scarcely ever more than half a 

 mile. Haidinger gives the full details of an extraordinary case, in which the thunder- 

 loud formed a stratum of only 25 feet thick, raised 30 yards above the ground. 

 Yet two people were killed on this occasion. Other notable instances of a similar 

 extreme character are recorded. 



Careful experiment shows us that the air is scarcely ever free from electricity, 

 even in the clearest weather. And even on specially fine days, when large separate 

 cumuli are floating along, each as it comes near produces a marked effect on the 

 electrometer. Andrews obtained by means of a kite, on a fine clear day, a steady 

 decomposition of water by the electricity collected by a fine wire twisted round the 

 string. Thanks to Sir W. Thomson, we can now observe atmospheric electricity in 

 a most satisfactory manner. I will test, to show you the mode of proceeding, the 

 air inside and outside the hall. [The experiment was shown, and the external air 

 gave negative indications.] 



On several occasions I have found it almost impossible, even by giving extreme 

 directive force to the instrument by means of magnets, to measure the atmospheric 

 potential with such an electrometer, and had recourse to the old electroscope, with 

 specially long and thick gold-leaves. On February 26th, 1874, when the sleet and 

 hail, dashing against the cupola of my class-room, made so much noise as to 

 completely interrupt my lecture, I connected that instrument with the water-dropper, 

 and saw the gold-leaves discharge themselves against the sides every few seconds, 

 sometimes with positive, sometimes, often immediately afterwards, with negative 

 electricity. Such effects would have required for their production a battery of tens 

 of thousands of cells. Yet there was neither lightning nor thunder, and the water 

 was trickling from the can at the rate of only two and a half cubic inches per 

 minute. Probably had there not been such a violent fall of sleet steadily discharging 

 the clouds we should have had a severe thunderstorm. That this is no fancied 

 explanation is evident from the fact that falling rain-drops are often so strongly 

 charged with electricity as to give a spark just before they touch the ground. On 

 such occasions, when the fall occurs at night, the ground is feebly lit up. This 

 "luminous rain," as it has been called, is a phenomenon which has been over and 

 over again seen by competent and trustworthy observers. In the Comptes Rendus 

 for November last we read of the curious phenomenon of electrification of the 



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