THUNDER-STORMS. 215 



cases, it is of no consequence to the motion of the 

 storm whether it be that the disturbed air comes 

 from the distant place, or a steady current comes 

 from that place, and acts upon disturbed air at the 

 other. But there is a very marked difference in the 

 change of the weather ; for if the disturbed air come, 

 it brings broken or rainy weather ; and if the steady 

 current come, it drives the bad weather away. If 

 it has been a tract of dry weather, and if curl-cloud 

 appears, and then a thunder-storm follows, we may 

 be sure of a tract of bad weather ; and if after con- 

 tinued alternations of showers and warmth, and cold 

 bleak winds, thunder ensues, we may be equally 

 certain that the weather will clear up. The former 

 case is, however, in the temperate latitudes, by far 

 the most frequent. The dry air in fine weather is a 

 much quicker conductor of heat than the moisture 

 in broken weather ; and when the earth is dry it 

 both reflects and radiates heat, whereas the wet 

 earth produces cold by evaporation. Besides, there 

 is no attraction of cohesion between the dry earth and 

 the water that forms a cloud ; while between the wet 

 earth or water, and that water, there is the very 

 same attraction of cohesion by which clouds are ac- 

 cumulated in the sky. The cloud thus comes down 

 to the moist surface, and avoids the dry ; and even 

 those thunder-showers that have their causes on the 

 surface of the earth follow thick leafy woods and 

 the courses of the rivers. Even in mountainous 

 countries, though the local thunder-clouds do some- 

 times strike the peaks, they much more frequently 

 plough up trenches in those elevated heights which 

 abound in moist peat earth, and are always saturated 

 with water. That (as indeed all the occurrences 

 which are perfectly natural are when we once un- 

 derstand them) is highly beneficial. Where the 

 cloud strikes it usually falls, and the water which 

 falls upon those heights does not so soon run to 

 waste as if it fell on the peaks. 



