302 PETER GUTHRIE TAIT 



form is not quite so easily explained. It is certainly destroyed, in the case of short 

 sparks, by heating the air. [Photographs of sparks in hot and in cold air were 

 exhibited. One of each kind is shown in the woodcut. The smoother is that which 

 passed through the hot air. The other passed through the cold air nearer the camera, 

 and is therefore not quite in focus.] 



Now heating in a tube or flame not only gets rid of motes and other combustible 

 materials but it also removes all traces of electrification from air. It is possible, 

 then, that the zig-zag form of a lightning flash may, in certain cases at least, be due 

 to local electrification, which would have the same sort of effect as heat in rarefying 

 the air and making it a better conductor. 



A remark is made very commonly in thunderstorms which, if correct, is 

 obviously inconsistent with what I have said as to the extremely short duration of 

 a flash. 



Even if we supposed the flash to be caused by a luminous body moving along, 

 like the end of a burning stick whirled around in a dark room, it would pass with 

 such extraordinary rapidity that the eye could not possibly follow its movements. 

 Hence it is clear that when people say they saw a flash go upwards to the clouds 

 from the ground, or downwards from the clouds to the ground, they must be mistaken. 



The origin of the mistake seems to be a subjective one, viz., that the central parts 

 of the retina are more sensitive, by practice, than the rest, and therefore that the 

 portion of the flash which is seen directly affects the brain sooner than the rest. 

 Hence a spectator looking towards either end of a flash very naturally fancies that 

 end to be its starting point. 



Before I can go farther with this subject it is necessary that I should give 

 some simple facts and illustrations connected with ordinary machine electricity. 

 These will enable you to follow easily the slightly more difficult steps in this part 

 of our subject which remain to be taken. 



Since we are dealing mainly with tnotion of electricity, it is necessary to 

 consider to what that motion is due. You all know that winds, i.e., motions of the 

 air, are due to differences of pressure. 



If the pressure were everywhere the same at the same level we should have no 

 winds. Similarly the cause of the motion of heat in a body is difference of 

 temperature. When all parts of a body are at the same temperature there is no 

 change of distribution of heat. Now electricity presents a precisely analogous case. 

 It moves in consequence of difference of potential. Potential, in fact, plays, with 



