THUNDER-STORM?. 



have when the clouds are in contact, then their approach and suh-cquent con- 

 tact will cause no change in their electrical state save what would he din- to 

 inductive action. Their charges after contact will be the same as In-fur.-. u< 

 electricity passing from either to the other. But if their electrical cli 

 have not this particular relation, then a new distribution of electricity wilTbe 

 the consequence of their mutual approach ; that which has less positive elec- 

 tricity than the condition of contact requires will receive the deficiency from 

 the other, and this change will be effected by an explosion before the" actual 

 contact of the clouds, in the same manner as the electrical equilibrium of two 

 conductors is established by the transmission of the spark before contact. 

 The distance at which the explosion'will take place, and its force, will di-prnd 

 on many circumstances, such as the difference between the actual charges of 

 ' } the clouds, and the charges due to contact, the form of the clouds, and the 

 state of the intervening atmosphere. 



It is evident, therefore, that an electrical explosion may take place between 

 iwo clouds, whether they are both similarly electrified, or oppositely electrified, 

 or one be electrified and the other in its natural state. 



As the ground is, in general, negatively, and the clouds positively elec- 

 trified, a discharge will take place between the clouds and the earth when the* 

 former approach the earth within such a distance that the force of the electri- 

 city shall overcome the resistance of the surrounding air. 



Since free electricity accumulates in great intensity at prominent points 

 of a conducting body, the negative electricity of the earth may be expected to 

 be most intense at mountain summits. Clouds being, in general, charged with 

 positive electricity, an attraction will, consequently, be exerted upon them 

 which, conspiring with the attraction of gravitation, will draw, them round such 

 summits. 



The mutual approach of two clouds oppositely electrified is promoted by the 

 attraction due to their electricities : but when two clouds are similarly electri- 

 fied they will repel each other and their approach must be due to contrary our- 

 rents of air passing through strata of the atmosphere at different elevations, by 

 which the clouds are brought one under the other. 



Beccaria, who observed at Turin, in Piedmont, in a country eminently fa- 

 vorable for such observations, being almost surrounded by lofty ranges, has re- 

 corded, with great precision, the appearances of the clouds precursive of a 

 storm. The observations of this philosopher being limited to the lower sur- 

 face of the clouds, M. Arago has obtained some accounts of the superior sur- < 

 face, from the military engineers employed in the trigonometrical survey, and < 

 who, being placed at elevated stations on the Pyrenees, were enabled to ob- , 

 serve the superior surface of the strata of clouds situated below them. From \ 

 the reports of these officers, and especially those of MM. Peytier aad Hos- 

 sard, it appears that there is no correspondence between the upper and 

 lower surface of a stratum of thunder-clouds ; that when the inferior sur- 

 face is perfectly even and level, the superior surface wiii bs broken in o 

 ridges and protuberances, rising upward to great altitudes, like '.he surface of 

 the earth in an alpine district. In times of great heat, such strata were ob- 

 served suddenly to send upward lofty vertical cones, which, stretching into 

 higher regions of the air, established, by their conducting power, an electrical 

 communication between strata of the atmosphere at very different heights. 

 This appearance was generally observed to precede a thunder-storm. 



Franklin, Saussure, and most other meteorologists, have agreed that thunder 

 never proceeds from a solitary, isolated cloud. Franklin states, that if a thun- 

 der-cloud be at any considerable distance from the zenith of the observer, so 

 as to be viewed obliquely, it will be apparent that there are, in every such case, > 



