366 WRITINGS OF JOSEPH HENRY. [18S5- 



twilight. Mr. Brooks, one of the directors of the telegraph 

 line between Pittsburg and Philadelphia, informs us that on 

 one occasion to satisfy himself on this point he asked for 

 information from a distant operator during the appearance 

 of flashes of this kind in the distant horizon, and learned that 

 they proceeded from a thunder-storm then raging two hun- 

 dred and fifty miles eastward of his place of observation. 



The third class is called "globular lightning," which is re- 

 markable (besides its peculiarity of form) for the slowness of 

 its motion. The occurrence of this form of lightning is very 

 rare, and were not the phenomenon well authenticated, we 

 should be inclined to regard it as a delusion. But it does not 

 comport with the cautious procedure of true science to deny 

 the existence of all appearances which may not come within 

 the prevision of what are considered as established princi- 

 ples. Although when facts of an extraordinary nature are 

 related to us, they should not be received with that easy cre- 

 dence which might be due to less remarkable phenomena, yet 

 after having fully satisfied ourselves of their reality, we must 

 endeavor to collect all the facts connected with them, and to 

 ascertain with accuracy the essential conditions on which 

 they depend. Arago has given a number of instances of 

 this remarkable form of the electrical discharge, the general 

 appearance of which is that of a ball moving slowly through 

 the air and sometimes when coming near a body, exploding 

 with tremendous violence. 



The only explanation which has been suggested for this 

 remarkable meteor, and which at first sight appears to be- 

 long entirely to some other class of phenomena than those 

 denominated electrical, is that which was in part suggested 

 (I believe) by Sir W. Snow Harris. According to his hypoth- 

 esis, the ball of light is the result of what is analogous to that 

 which is known as a glow discharge, a phenomenon familiar 

 to all who are in the habit of making electrical experiments. 

 When a conductor connected with the earth is brought near 

 a charged body, particularly when the air is damp, a partial 

 silent discharge will take place, during which (although 

 there may be no light perceptible in the space between the 



