ELECTRICITY. 15S 



northern lights, as they are sometimes called, are 

 most frequent in the countries nearest the poles of 

 the earth. In Greenland and Iceland they are 

 sometimes so strong and vivid, as to afford light 

 sufficient to read by. A very beautiful experi- 

 ment to illustrate this is what is called the luminous 

 conductor*, represented in Plate 20. fig. 3. A is a 

 glass tube of about two inches in diameter, and 

 two feet long, capped at both ends with brass, 

 having one of the ends furnished with a stop-cock, 

 and a screw to fit into the plate of the air-pump. 

 This tube is exhausted of air, and when it is placed 

 in the circuit of the electric fluid, by fixing a 

 chain to each end which is connected with the 

 positive and negative part of the machine, the elec- 

 tricity, in passing through it, exhibits a beautiful 

 luminous appearance, very much resembling the 

 aurora borealis. 



The balls of fire, as well as the shooting stars 

 occasionally seen in the air, seem to be masses of 

 electricity at so great a distance that their angular 

 velocity is not sufficient to prevent the eye from 

 discerning their shape. It is probable, that every 

 electric spark, or flash of lightning, consists of one 

 or more balls of fire, though their extreme velocity 

 presents them to the eye under the form of a line, 

 or lines. 



The appearance of water-spouts is generally con- 

 nected with electric phenomena. Their immediate 

 cause appears to be a vacuum that is formed in a 

 part of the air by the convergence of winds, and 

 their consequent whirling motion. When this 

 happens over the sea, the water is forced up by the 

 pressure of the surrounding atmosphere, and kept 

 suspended until the cause that produced it ceases. 



