On the Oscillations of Lightning Discharges. 343 



wires to the two pairs of quadrants of my quadrant electro- 

 meter, and it is generally convenient to allow the lowest to lie 

 uninsulated on an ordinary table and to connect it metallically 

 with the outer case of the electrometer. 



To make an experiment, (1) connect the two fine wires 

 metallically, and let the electrometer-needle settle to its 

 metallic zero. 



(2) Break the connexion between the two fine wires, and 

 let a weight of a few hektogrammes or kilogrammes fall from 

 a height of a few millimetres above the upper plate and rest 

 on this plate. A startlingiy great deflexion of the electrometer- 

 needle is produced. The insulation of the india-rubber 

 supports and of the quadrants in the electrometer ought to 

 be so good as to allow the needle to come to rest, and the 

 steady deflexion to be observed, before there is any con- 

 siderable loss. 



If, for example, the plates are placed with their zinc faces 

 up, the application of the weight causes positive electricity to 

 come from the lower face of the uppermost plate and deposit 

 itself over the upper surface of plate and weight, and on the 

 electrode and pair of quadrants of the electrometer connected 

 with it. 



XXXV. On the Oscillations of Lightning Discharges and of 

 the Aurora Borealis. By John Trowbridge*. 



[Plate III.] 



IT is well known that when air is subjected to a sudden 

 strain at the moment of an electrical discharge, it acts like 

 glass or a similar elastic solid and is cracked in zigzag fissures ; 

 indeed the resemblance between the ramifications of lightning 

 and the seams produced in plates of glass by pressure has 

 been commented upon by various observers. Photographs of 

 powerful electric sparks lead one to conclude that a discharge 

 of lightning makes way for its oscillations by first breaking 

 down the resistance of the air by means of a disruptive pilot 

 spark : through the hole thus made in the air the subsequent 

 surgings or oscillations take place. 



In examining the early photographs, taken by Feddersen, of 

 electric sparks, one perceives indications that the electric oscil- 

 lations tend to follow, for at least some hundred-thousandths 

 of a second, the path made by the pilot spark ; and there are 

 observers who believe that by rapidly moving a camera they 



* Communicated by the Author. 



