-1859] WRITINGS OF JOSEPH HENRY. 393 



in London. It is true, that when a needle is presented to a 

 charged conductor, the electricity is drawn off silently with- 

 out an explosion, and this is always the case if sufficient time 

 be allowed for the electricity to escape in this way. But if 

 the point be suddenly brought within striking distance of 

 the conductor by a rapid motion, such as would be produced 

 by the movement of a horizontal arm carrying the point im- 

 mediately under the conductor in an instant, an explosive 

 discharge will take place. In this case sufficient time is not 

 given for the slower transmission of the electricity by what 

 has been denominated the glowing discharge, and a rupture 

 of the air is produced as in the action of a conductor termi- 

 nated by a ball. 



It would follow from this that in the case of a rapidly-mov- 

 ing cloud across the zenith of a rod, there would be a greater 

 tendency to an explosive discharge on the point than when 

 the cloud was nearly stationary. For a similar reason, if a 

 point connected with the earth by a wire be directed toward 

 an insulated conductor, and the latter be suddenly electrified 

 by a discharge from a second conductor, an explosion will 

 take place between the first conductor and the point. A 

 similar effect would be produced if a lower cloud received 

 a sudden discharge from one above it, a case which prob- 

 ably frequently occurs in nature. Mr. Wise informs us that 

 when a discharge takes place from the base of a cloud to the 

 earth, a discharge is seen to pass between the upper and 

 lower part of the cloud. (A condition shown in Fig. 19.) We 

 are warranted from the foregoing facts, as well as from the 

 numerous examples in which lightning has actually been 

 seen to fall upon pointed rods explosively, and the number 

 of points which have been melted, to conclude that the rod 

 under certain conditions does actually attract the lightning, 

 though when properly constructed it transmits it without 

 disturbance to the earth. 



It has been denied by some that the point has any per- 

 ceptible influence in lessening the number of strokes from 

 a cloud, but this proposition can scarcely be doubted when 

 we reflect upon the fact that it is not necessary to entirely 



