THUNDER AND LIGHTNING 211 



the sudden volleying ascent of a column of warm moist air 

 to the cooler heights, and its consequent condensation. They 

 perpetuate the momentary form of a volume of steam shot 

 from the funnel of a railway engine. Such clouds are not 

 always accompanied by thunder; but when these unstable 

 exchanges of the atmosphere are in progress thunder-storms 

 are very likely to occur, especially in sultry and windless 

 weather. Then, in the absence of a steady breeze to carry 

 off the heated air from near the earth's surface, it rises into 

 the colder air above ; and, in some way not clearly under- 

 stood, owing to this displacement the upper layers of the 

 atmosphere become highly charged with electricity, and 

 flashes of lightning burst forth between layer and layer of 

 cloud, or between the cloud and the earth. The thunder- 

 storm goes on until the accumulation of electricity is evenly 

 distributed again. By this time, in a normal and wholesome 

 thunder-storm, the air is freshened by an inrushing current, 

 and probably by the electric discharges themselves ; vegeta- 

 tion is revived by the rain accompanying the storm, and by 

 the peculiar tonic quality in the air after it has passed. The 

 massive cumulus clouds have broken up with the redistribu- 

 tion of the air currents, and the whole aspect of the heavens 

 is changed ; either there is a clear blue sky, or, if the rain has 

 been heavy, there is often a cool grey canopy of mist which 

 remains till evening, and dissolves in the course of the night 

 Summer storms take place most often in the afternoon, 

 when the heat is normally greatest, or at any rate there is 

 the largest accumulation of heated air brooding over the 

 earth. Between three and five o'clock is the commonest 

 time, and then a serene and refreshing evening may follow. 

 The proverbial gibe at the English summer is really a testi- 

 monial to its good qualities ; for if we could always have 

 1 three fine days and a thunder-storm ' the crops and flocks 



