246 Natural History. [Chap. III. 



portant points. There are, probably, no two indi- 

 viduals to whom the scientific world is more in- 

 debted for the minuteness, the accuracy, and the 

 success, of their meteorological investigations, than 

 to tliese philosophers of Geneva. * 



All our knowledge of Atmospherical Electricity 

 is the product of the eighteenth century. To this 

 subject the attention of philosophers has been par- 

 ticularly drawn since the time of Dr. Franklin's 

 discovery that lightning and thunder are occasion- 

 ed by the agency of electricity. 



The most complete set of experiments on this 

 part of meteorology were made by professor Bec- 

 caria, of Turin. lie found that the air is almost 

 always positively electrical, especially in the day- 

 time, and in dry weather ; that when dark or wet 

 weather clears up, the electricity is always nega- 

 tive ; and that low thick fogs, rising into dry air, 

 parry up a great deal of electric matter. He as- 

 certained that the mid-day electricity of days 

 equally dry is always proportional to the heat ; 

 that winds lessen the electricity of a clear day, 

 especially if damp; and that, for the most part, 

 when there is a clear sky, and little wind, a consi- 

 derable quantity of electricity arises after sun-set, 

 at dew-falling. Considerable light has been thrown 

 on the sources of atmospherical electricity, by the 

 experiments of M. de Saussure and other minera- 

 logists. Air is not only electrified by friction, like 

 other electric bodies, but the state of its electricity 

 is changed by various chemical operations which 

 often go on in the atmosphere. Evaporation seems* 

 in all cases to convey electric matter into the atmo- 

 .sphere ^ and de Saussure has ascertained, that the 



