THUNDER-STORMS. 541 



Thunder-storms have been scientifically studied in various 

 countries, and the broad fact has been elicited that they travel 

 over the earth's surface like wind-storms, but at a higher velocity. 

 To give an idea of this, I may quote some statements made be- 

 fore the Royal Meteorological Society last June, in relation to 

 the storm of the 2d of that month. This storm progressed from 

 Wiltshire to Edinburgh, over a distance of four hundred miles, at 

 a nearly uniform speed, the rate of travel being about fifty miles 

 an hour. This is an unusually rapid rate of advance for a wind- 

 storm over these islands. I am not speaking of the velocity of 

 the wind in the storm, but of the velocity of the storm system as 

 a whole. In this storm many of the hailstones which were col- 

 lected weighed over an ounce. Some at Docking, near King's 

 Lynn, were said to be three inches in diameter, and to weigh 

 three and a half ounces. One was weighed at Barden Mill, near 

 Tunbridge Wells, and was said to turn the scale at half a pound. 

 As regards the incessant character of the lightning in London, 

 one observer at Highgate counted twelve hundred and forty-four 

 displays during the two hours ending at 11.10 P. M., giving an 

 average of over ten per minute. Another observer, at Westgate- 

 on-Sea, gave a much higher figure for frequency ; his attempt to 

 count breaking down at the very high number of one hundred 

 and thirty-one per minute. 



Thunder-storms are much more frequent in low latitudes than 

 in high. In some tropical countries they are said to occur regu- 

 larly every afternoon. At Rio the story was that at certain sea- 

 sons, in issuing invitations to afternoon parties, it was usual to 

 specify whether guests were to assemble before or after the 

 thunder-storm. In Abyssinia, D'Abbadie gives, as the average of 

 four years, 410*6 as the annual number of these storms. Many of 

 these, however, consisted of only one or two flashes of lightning. 

 It was formerly believed that such storms never were noticed in 

 the arctic regions, but this is not the case, for one was experi- 

 enced at Bell Sound, Spitsbergen, in 78° north latitude, in August, 

 1873 ; and a succession of thunder-storms was reported for several 

 days in July, 1870, on the west coast of Nova Zembla. At any 

 rate, in such high latitudes they are very rare. 



Thunder-storms are generally divided into two groups — heat 

 thunder-storms and cyclonic thunder-storms. The former are the 

 summer type, while the latter occur principally in autumn and 

 winter. We may also say that the former are essentially conti- 

 nental, while the latter are characteristic of the ocean or island 

 climate. In Iceland all the thunder-storms are of this latter type, 

 and occur in winter. The same conditions show themselves on the 

 British Atlantic coasts, where there is a decided maximum of fre- 

 quency of such storms in winter, even in the latitude of the south- 



