360 



WRITINGS OF JOSEPH HENRY. 



[1855- 



tremity, leaving a deficiency in the upper part. Now if we 

 admit (agreeably to the theory of Mr. Espy,) that a cloud re- 

 sults from the upward motion of a mass of moist and heated 

 air, the vapor of which is condensed as it ascends into the 

 colder regions, thus forming a high perpendicular column of 

 partially conducting material, it will be evident that by induc- 

 tion, the upper part of this cloud will become negatively elec- 

 trified, and the lower part positively, as in the case of the con- 

 ductor, Figure 15. The intensity of this excitement will de- 

 pend upon the length of the vertical dimensions of the 

 cloud, (which in many cases is exceedingly great,) and also 

 upon the density, and consequentl}^ the conducting power of 

 the vapor. The induction of the earth being very intense, a 

 partial excitement of the atoms of vapor may take place even 

 before the condensation of the whole mass has reached its 

 maximum. If this be the case, a transparent mass of vapor, 

 or that which is merely beginning to condense into cloud, 

 will be electrified throughout its entire mass ; and when the 

 condensation of the 

 vapor has gone so 

 far as to render the 

 interior a tolerably 

 good conductor, the 

 electricity of each 

 atom will be re- 

 pelled to the sur- 

 face, as in the case 

 of a globular con- 

 ductor; the intensi- 

 ty will thus be high- 

 ly increased, and 

 while the rushing 

 upward of moist air 

 is going on, a series 

 of discharges will 

 take place between 

 the upper and lower 

 portions of the cloud. Fig. 19. 



