SUMMER GUSTS. 337 



the air as it fills the vacuum produced by the 

 passage of the electric bolt through it ; or by its 

 violent collision against the air, causing a tremour 

 of the atmosphere to the distance of many miles, 

 which is communicated to buildings and other 

 solid bodies. It is likewise probable that the 

 luminosity of the spark is owing to a sudden 

 combustion of air or vapour by so intense a heat. 



It was first observed by Dr. Franklin, that the 

 cold summer gusts of the middle latitudes, gene- 

 rally come from the west ; from which he con- 

 cluded, that they were caused by a descent of 

 cold air from the upper current of the atmos- 

 phere, on its passage from the equatorial to the 

 polar latitudes. That this is chiefly the cause 

 of our prevalent west winds, during about two- 

 thirds of the year in the middle latitudes, is ob- 

 vious from what was stated, page 322. 



That this is also the true mode of accounting 

 for many of our thunder storms, would appear 

 reasonable from the following considerations. 

 They occur during the most sultry weather, and 

 hottest time of the day; generally the afternoon, 

 when the lower atmosphere is greatly rarefied, 

 so as to favour a descent of cold air from above, 

 which rapidly condenses the transparent vapour 

 of the lower atmosphere into floods of rain, that 

 are often attended with hail, and always with a 

 great reduction of temperature. In this way 

 clouds are often formed suddenly, the whole sky 



