THUNDER STORMS. 



the surface of the earth, such as one place scorched 

 to almost absolute dryness, while another retains its 

 average degree of moisture. A large city, a barren 

 moor, or an arid down, may, in very hot weather, 

 which has been long continued, be the means 

 of producing those motions in the air, the result of 

 which is a thunder storm ; but thunder storms that 

 have that origin are generally very local, and of 

 short duration. 



If the storm has its origin in the upper regions 

 of the air, its primary cause must be at a greater 

 distance, and consequently more powerful, as it 

 can propagate its action through a greater volume 

 of air. The storm itself is therefore more widely 

 extended, and of longer duration ; and indeed it 

 generally brings a change of the weather. In 

 those* 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 differ- 

 ence in the change of the weather ; for if the dis- 

 turbed 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 wea- 

 ther, and if curl-cloud appears, and then a thunder 

 storm follows, we may be sure of a tract of bad 

 weather; and if after continued 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 



