43 



NATURE 



\Scpt. 9, i8So 



currents is strongly marked in almost every great thunder- 

 storm, and is precisely analogous to that observed in 

 the centre of a West Indian tornado and of a Chinese 

 t\]jhoon. 



When any portion of the atmosphere is ascending it 

 must be because a denser portion is descending, and 

 whenever such motions occur ivitlt acceleration the 

 pressure must necessarily be diminished, since the lower 

 strata are not then supporting the whole weight of the 

 superincumbent strata. If their whole weight were sup- 

 ported they would not descend. Thus even a smart 

 shower of rain must directly tend to lower the barometer. 

 [A long glass tube, filled with water, was suspended in a 

 vertical position by a liglit spiral spring, reaching to the 

 roof of the hall. A number of bullets hung at the top of 

 the water column, attached to the tube by a thread. 

 When the thread was burned, by applying a lamp, the 

 bullets descended in tlie 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 thunderstorms, 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 man)- 

 cases, merely the faint eftect of a distant thunderstorm, 

 but which has also been observed when the sky appeared 

 tolerably clear, and when it was certain that no thunder- 

 storm 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 Stew.irt 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 con- 

 siderable 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 1 have two re- 

 marks 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 precautions 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. 



PHYSICS WITHOUT APPARATUS 

 IV. 

 'T'HE science of heat constitutes one of those depart- 

 ■*■ ments of physics in which both the uninitiated 

 beginner and the advanced student can find food for 

 thought. To follow out the theoretical teachings of the 

 science of heat requires a knowledge of abstruse mathe- 

 matical formula;; but, on the other hand, a very large 



' Continued from p. 368. 



proportion of the fundamental facts of experiment upon 

 which the science depends can be illustrated with the 

 simplest means. 



The property possessed by almost all material bodies 

 of expanding when they are warmed affords us the means 



of ascertaining the degree to which they are warmed. 

 Thus the expansion of the quicksilver in the bulbs of our 



