THE LIGHTNING FLASH 301 



absolutely fixed when illuminated by its light alone. One can read by its light a 

 printed page stuck on a disc revolving at great speed. But the most severe test is 

 that of Sir Charles Wheatstone's revolving mirror. Seen by reflection in such a 

 mirror, however fast it may be rotating, a flash of lightning is not perceptibly 

 broadened, as it certainly would be if its duration were appreciable. 



The apparatus which, in our laboratories, enables us to measure the time which 

 light, moving at nearly 200,000 miles per second, takes to pass over a few feet, is 

 required to prove to us that lightning is not absolutely instantaneous. Wheatstone 

 has shown that it certainly lasts less than a millionth part of a second. Take this, 

 along with Swan's datum, which I have just given you, and you see that the apparent 

 brightness of the landscape, as lit up by a lightning flash, is less than one hundred 

 thousandth part of what it would be were the lightning permanent. We have thus 

 rough materials for instituting a comparison between the intrinsic brightness of 

 lightning and of the sun. 



Transient in the extreme as the phenomenon is, we can still, in virtue of the 

 duration of visual impressions, form a tolerably accurate conception of the form of 

 a flash ; and in recent times instantaneous processes of photography have given us 

 permanent records of it. These, when compared with photographic records of ordinary 

 electric sparks, bear out to the full the convictions at once forced by appearances on the 

 old electricians, that a flash of lightning is merely a very large electric spark. The 

 peculiar zig-zag form, sometimes apparently almost doubling back on itself, the 

 occasional bifurcations, and various other phenomena of a lightning flash, are all shown 

 by the powerful sparks from an electric machine. [These sparks were exhibited 

 directly ; and then photographs, of which one is represented in the woodcut, were 

 exhibited.] 



But the spectroscope has recently given us still more convincing evidence of 

 their identity, if any such should be wanted. This is a point of great importance, but 

 I have not now time to discuss it. Thus, though on a very small scale comparatively, 

 we may study the phenomena of lightning at convenience in our laboratories. We 

 thus know by experiment that electricity chooses always the easiest route, the path 

 of least resistance. Hence the danger of taking the otherwise most natural course of 

 standing under a tree during a thunderstorm. The tree, especially when wet, is a 

 much better conductor than the air, and is consequently not unlikely to be chosen 

 by the discharge ; but the human body is a conductor much superior to the tree, 

 and therefore is chosen in preference so far as it reaches. The bifurcations of a flash 

 can puzzle no one who is experimentally acquainted with electricity, but the zig-zag 



