3 2o PETER GUTHRIE TAIT 



water, and during their descent the spring contracted so as to raise the whole tube 

 several inches.] 



In what I have said to-night I have confined myself mainly to great thunder- 

 storms, and to what is seen and heard by those who are within their sphere of 

 operation. I have said nothing of what is commonly called summer-lightning, which 

 is probably, at least in a great many cases, merely the faint effect of a distant 

 thunderstorm, but which has also been observed when the sky appeared tolerably 

 clear, and when it was certain that no thunderstorm of the ordinary kind had occurred 

 within a hundred miles. In such cases it is probable that we see the lightning of a 

 storm which is taking place in the upper strata of the atmosphere, at such a height 

 that the thunder is inaudible, partly on account of the distance, partly on account 

 of the fact that it takes its origin in air of small density. 



Nor have I spoken of the aurora, which is obviously connected with atmospheric 

 electricity, but in what precise way remains to be discovered. Various theories have 

 been suggested, but decisive data are wanting. Dr Balfour Stewart inclines to the 

 belief that great auroras, visible over nearly a whole terrestrial hemisphere, are due 

 to inductive effects of changes in the earth's magnetism. This is not necessarily 

 inconsistent with the opinion that, as ordinary auroras generally occur at times when 

 a considerable change of temperature takes place, they are phenomena due to the 

 condensation of aqueous vapour in far less quantity, but through far greater spaces, 

 than the quantities and spaces involved in ordinary thunderstorms. 



In taking leave of you and of my subject I have two remarks to make. First, 

 to call your attention to the fact that the most obscure branches of physics often 

 present matter of interesting reflection for all, and, in consequence, ought not to be 

 left wholly in the hands of professedly scientific men. Secondly, that if the pre- 

 cautions which science points out as, at least in general, sufficient, were recognised 

 by the public as necessary, the element of danger, which in old days encouraged 

 the most debasing of superstitions, would be all but removed from a thunderstorm. 

 Thus the most timid would be able to join their more robust fellow-creatures in 

 watching fearlessly, but still of course with wonder and admiration, one of the most 

 exquisite of the magnificent spectacles which Nature from time to time so lavishly 

 provides. 



