THUNDER-STORMS. 543 



" summer lightning," takes place without any thunder ; so that, 

 in such cases, no actual thunder-storm is in progress. 



3. " Globular Lightning." — This is a rare phenomenon, and one 

 which no one has as yet been able to produce in the laboratory, 

 whereas the phenomena of the two previous types are easily pro- 

 duced. The general description of the occurrence is that a lumi- 

 nous ball is seen, moving very slowly, not touching any object, 

 and eventually breaking up with a violent explosion and the ap- 

 pearance of several flashes of ordinary lightning. It is reported 

 that persons have gone out from a house into a street to follow 

 such a ball and watch its movements, so that the occurrence must 

 have lasted at least a number of seconds. Ordinary lightning, as 

 is well known, is practically quite instantaneous. The size of the 

 ball on different occasions has varied from that of an orange to 

 that of a large glass lamp-globe, or even larger. Many physicists 

 refuse to believe any accounts of this manifestation of the elec- 

 trical discharge, but the reports of it are too numerous and cir- 

 cumstantial for us to consider them to be entirely baseless. 



There is another way of classifying lightning flashes, and that 

 is as to their color. The seven colors of the solar spectrum are 

 well known, but the spectrum of the electric spark differs mate- 

 rially from the solar spectrum. It exhibits rays which extend far 

 beyond the extreme violet of the solar spectrum. We see, there- 

 fore, that in the light of lightning a wide range of color is pos- 

 sible. If any of my readers have ever watched a storm carefully, 

 they must have noted that some of the flashes were bluish, others 

 reddish, etc. It is generally the blue tints which accompany the 

 most destructive strokes. 



Some attempts have been made to estimate the actual force 

 exerted by a lightning flash. The late Mr. de la Rue constructed 

 a magnificent electrical battery of many thousand cells. From 

 experiments with this, the number of cells being raised to 15,000, 

 and the " potential " of each being rather over one " volt," it was 

 found that 9,700 " volts " — say 9,500 cells — were required to pro- 

 duce a discharge through one centimetre ('3937 inch). Starting 

 from these data, the electro-motive force requisite to produce a 

 flash of lightning one mile (63,360 inches) in length, at ordinary 

 pressures, is 1,480,570,000 volts, practically given by a battery of 

 fifteen hundred million cells. 



A flash a mile in length is nothing very extraordinary, and it 

 is therefore not to be wondered at that experiments to bring 

 electricity down from the clouds are very dangerous, and have 

 frequently had fatal results. Soon after Franklin, in the last 

 century, had made his famous experiment with a kite, and proved 

 that electricity existed in a thunder-cloud, natural philosophers 

 generally began to imitate him. One of them in St. Petersburg, a 



