THUNDER-STORMS. 



511 



Before the concurrent force of this evidence all doubt as to the reality of ball- 

 ) lightning must disappear. 



But while on the one hand we are compelled to admit that such phenom- 

 ; eria do occur, and that they are true electrical effects, on the other hand we 

 ( are no less compelled to trace in them the characters of a different kind of 

 j electrical discharge from the ordinary lightning flash. Professor Faraday divides 

 the forms of discharge into the spark, the brush, and the glow. The glow is 

 most readily obtained in the rarefied air of a partially exhausted receiver ; and 

 differs from the brush in being due to a constant renewal of discharge instead 

 of an intermitting action. Now Mr. Snow Harris suggests in his recent Trea- 

 tise, on Thunder Storms, p. 38, that the ball discharge in question possesses 

 many features of resemblance to the glow ; and in addition it possesses motion. 

 The latter fact is readily accounted for, inasmuch as the cloud which causes the 

 discharge is always progressing. The transition from the glow to the spark, or 

 flash, is easily explained ; for when the cloud passes over any terrestrial ob- 

 ject by which the resistance to discharge is reduced within the striking dis- 

 tance, disruptive discharge must take place ; the glow remaining only so long 

 as the resistance opposed the actual flash. Such a ball discharge is described 

 as having approached the ship " Montague," and to have exploded on the top- 

 mast ; and this is just what Mr. Harris's theory would lead us to expect. Am 

 there is reason to believe that many of the cases before us are not to be classed 

 among the effects of lightning. We shall again advert to this. 



ON THE SPEED OF LIGHTNING. 



The solution of this problem is due to Wheatstone, and like oome other re 

 suits of physical inquiry, such as the abstraction of lightning from the clouds, 

 \\ Inch was effected by a boy's kite, and the iridescent effect due to the varying 



