62 



are characterised by variable winds, oscillating from south- 

 west either to south-east or to north-west, combined with a 

 humid atmosphere, that thunderstorms are most frequent. 

 In such states of weather, and with such winds, which always 

 bring a good deal of moisture, thunderstorms often have a 

 kind of periodicity, recurring regularly at about the same 

 hour for several days in succession. This hour will be gene- 

 rally coincident with the hottest time of the day, it being then 

 " that the quantity of moisture evaporated from the ground, 

 and taken up by the ascending current into the region of the 

 clouds, attains its maximum, while there is a corresponding 

 accumulation of electricity arising from its 'condensation."* 



That thunderstorms are not necessarily connected with hot 

 weather, but that they are more directly due to changed 

 conditions of the atmosphere, caused either by a shift of the 

 wind, or by the ascending current (though from the latter 

 cause they never occur except in summer), is shown by their 

 not unfrequently taking place in early spring, and occasionally 

 even in mid-winter. 



In Bath, last summer, during the three hot months of May, 

 June, and July, there were only three heavy thunderstorms, 

 one in each of those months. The first occurred on the 29th 

 of May, — the same day on which there was one at Green mch, 

 as stated above from Mr. Glaisher ; the second on the 19th 

 of June ; and the third on the 15th of July. In the first case, 

 the storm was coincident with the passing of the wind from 

 south, through south-west, to north-west ; in each of the two 

 last cases with the passing of the same from north-east to 

 south-east and south. In all three months, irrespective of 

 the direction of the mnd, the atmosphere was in a very dry 

 state, some days — in May more particularly — remarkably so, 

 as noticed by observers in several places. Both in May and 



* See my "Observations in Meteorology," p. 293. 



