-1859] WRITINGS OF JOSEPH HENRY. 357 



sequently a portion of the natural electricity is driven into 

 the upper ball and the leaves themselves diverge with a 

 negative charge: the condition being opposite to that shown 

 at C. The writer had the pleasure in 1837 of witnessing this 

 interesting experiment as performed on a dry clear day by 

 Peltier himself. 



In order that the result may be shown with a slight change 

 of elevation it is necessary that a large ball be employed, so 

 that the effect may be multiplied by all the electricity of the 

 greater surface. When the electroscope is terminated with 

 the point of a fine needle, (though this is the best means of 

 attracting electricity from the air at a distance,) no effect 

 will be exhibited, provided the weather is dry and the sky 

 cloudless. 



From these experiments it appears evident that the 

 positive electricity with which the air is apparently always 

 charged in dry and clear weather, is not due to the free elec- 

 tricity of the atmosphere, but to the induction of the earth 

 on the conducting materials of which the instruments are 

 in whole or in part composed. 



It is not difficult to deduce from the same general princi- 

 ples the apparent changes in the electrical state of the atmos- 

 phere at different times of the day and in different hygro- 

 metrical conditions of the air. Vapor of water mingled with 

 the atmosphere renders the latter a positive conductor; and 

 when the moisture of the air extends up as high as the upper 

 part of the apparatus in Fig. 16, feeble negative electricity 

 will by slow conduction be diffused through the adjacent 

 strata, which acting upon the ball a will lessen the effect of 

 the more intense action of the earth. While the latter 

 tends to draw the natural electricity of the conductor down 

 into its lower part and to render the upper end negative, 

 the vapor around the ball will tend to draw it slightly up- 

 ward and thus diminish the effect, and lead the casual 

 observer to suppose that the air is less positively electrified. 

 Peltier in this way has shown (as well as Quetelet and Dell- 

 nian) that the variations of the electricity of the atmosphere 

 observed from day to day, and at different times in the 



