630 Prof. W. M. Thornton on Thunderbolts. 



certain already than his assumptions, for we have far more 

 evidence concerning the variation of the mass of an electron 

 with its velocity relative to the measuring instruments than 

 we have of the truth of the " law " of the conservation of 

 momentum as applied to the interaction of two systems 

 moving relative to each other with velocities approaching 

 that of light. But his logical methods are not justifiable. 

 A great part of the confusion into which the subject of the 

 motion of electric charges was plunged before the principle 

 of relativity came to enlighten us was due to a confusion 

 between logical proof and the mere extension of experimental 

 results to regions where there was no experimental evidence. 

 It is desirable to avoid such confusions for the future. 



Summary, 



The paper contains an examination of certain conclusions 

 reached by Prof. Lewis and Dr. Tolman, and by the latter 

 author alone. It is concluded (1) that the result attained by 

 the joint, authors is of much less generality than is claimed 

 by them, and (2) that the application of the result to prove 

 other propositions is unjustifiable, both on this ground and 

 on the ground of a general confusion between absolute and 

 relative velocity. 



Leeds, March 12, 1911. 



LXXIII. On Thunderbolts. By W. M. Thornton, D.Sc, 

 D.Eng.y Professor of Electrical Engineering in Armstrong 

 (College, Newcastle-upon- Tyne *. 



r |^HE term thunderbolt is given in common use both to the 

 JL rare phenomenon of ball lightning and to meteoric 

 stones. In the latter case it only has meaning, in so far as 

 their luminous path resembles lightning or that they cause 

 great atmospheric disturbance, and it is here used to describe 

 the former. The singularity of ball lightning lies in the com- 

 plete isolation of a gaseous sphere having no envelope, yet 

 within which there is energy stored by previous electrical 

 action. This is in the end liberated with explosive violence. 

 From the scattered records of its appearance the following 

 facts may be regarded as established. Jt is observed as a 

 luminous blue ball, occurring after lightning flashes of great 

 intensity, and either falling slowly from clouds or moving 



* Communicated bv the Author. 



