298 PETER GUTHRIE TAIT 



college so recently as to have been taught this. Yet, from the purely scientific 

 point of view, it is one of the most remarkable advances made during this century. 



But to know what electricity is, in the same sense as we may be said to know 

 what light is, does not necessarily guide us in the least degree to a notion of its 

 source in any particular instance. What, for example, causes the luminosity of 

 fireflies, glowworms, etc., among natural objects, and of phosphorescent watch-dials 

 among artificial ones ? The answer is not yet ready, though it may soon come. 

 So we might know quite well what is electricity and yet be, as I told you at 

 starting, we are, almost entirely uncertain of the exact source of atmospheric electricity. 



To come to my special subject. I am not going to try to describe a thunder- 

 storm. First, because I am certain that I could not do it without running the 

 risk of overdoing it, and thus becoming sensational instead of scientific ; and 

 secondly, because the phenomenon must be quite familiar, except perhaps in some 

 of its more singular details, to every one of you. From the artistic point of view 

 of the poets, who claim the monopoly of the expressions of wonder and awe, you 

 have descriptions without end. Who does not know at least the finest of them, 

 from that of Lucretius of old to that of Byron in modern times ? 



But science has to deal with magnitudes which are very much larger or smaller 

 than those which such words as huge, enormous, tiny, or minute are capable of 

 expressing. And though an electric spark, even from our most powerful artificial 

 sources, appears to the non-scientific trifling in comparison with a mile-long flash of 

 lightning, the difference (huge, if you like to call it) is as nothing to others with 

 which scientific men are constantly dealing. The nearest star is as much farther 

 from us than is the sun, as the sun is farther from us than is London. The sun's 

 distance is ninety-three millions of miles. If that distance be called enormous, and 

 it certainly is so, what adjective have you for the star's distance? The particles of 

 steam are as much less than a drop of water as an orange is less than the whole 

 earth. How can you fitly characterise their smallness? Ordinary human language, 

 and specially the more poetic forms of it, were devised to fit human feelings and 

 emotions, and not for scientific purposes : for the Imagination, not for the Reason. 

 As there is a limit alike to pleasure and to pain, so there is to wonder and 

 astonishment ; and with words expressive of something near these limits ordinary 

 language rests content. A thoroughly scientific account of a thunderstorm, if it were 

 possible to give one, would certainly be at once ridiculed as pedantic. 



Let us therefore, instead of attempting to discuss the phenomenon as a whole, 

 consider separately some of its more prominent features. And first of all, what are 

 these features when we are in the thunderstorm ? 



By far the most striking, at least if the thunderstorm come on during the day, 

 is the extraordinary darkness. Sometimes at midday in summer the darkness 

 becomes comparable with that at midnight, and in another sense it much resembles 

 midnight darkness, for it is very different in kind as well as intensity from that 

 produced by the densest fog. Objects are distinctly visible through it at distances 

 of many miles, whether when self-luminous or when instantaneously lit up by 

 lightning. The darkness then is simply intense shadow, produced by the great 

 thickness and great lateral extension of the cloud-masses overhead This altogether 



