1859] WRITINGS OF JOSEPH HENRY, 353 



condition the un-saturated matter in C, D will act upon 

 each atom of the fluid in the conductor, and tend to draw 

 the whole down into the lower bulb; the atoms at a will not 

 only be attracted downward by the action of the earth on 

 itself, but also pressed downward by the attraction of the 

 earth on all the atoms above it, and hence the intensity of 

 the electricity of the lower part of the conductor will be in- 

 creased by an increase in the perpendicular length of the 

 rod. Now, if we connect the lower bulb of the rod with the 

 earth by means of a good conductor, the redundant electricity 

 of the lower end will be drawn off into the earth and will 

 no longer re-act by its repulsion on the electricity of the rod 

 to drive it back into the upper bulb, and hence this will 

 become intensely negative, and in this condition it will be 

 a salient point on the surface of the earth. If while the 

 apparatus is in this condition we could touch the upper ball 

 with an electroscope it would exhibit a negative charge. 



If a conductor 20 feet in length were made to revolve on 

 a horizontal axis, passing through the middle of its length 

 so that it could be immediately changed from a horizontal 

 to a vertical position, any change in the apparent condition 

 of the atmosphere would be shown by the greater or less in- 

 tensity of the balls, as in succession they passed the lower 

 point of their circuit ; and an apparatus in the form of radia- 

 ting conductors like the spokes of a wheel, if made to revolve, 

 would furnish a constant source of electricity. An apparatus 

 of this kind was constructed by M. Palmieri, of Italy, and 

 might be used perhaps with success in studying the condi- 

 tion of the atmosphere in ascensions. 



The most convenient apparatus however for exhibiting 

 electricity by the induction of the earth is that invented by 

 M. Dellman, and shown in Fig. 16; which consists of a 

 large brass ball a supported on a thick brass stem — held 

 insulated inside of a glass tube by passing through corks of 

 gum shellac. The apparatus is fastened to a pole which is 

 temporarily elevated into the air by a windlass or the hand, 

 on the top of a house. When it reaches the height intended, 

 the wire h, connected with the earth below, is pulled, the end 



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