1895.] on Atmospheric Electricity. 469 



air." Whatever the special feeling of " thunder " may be due to, it 

 cannot be an electrical effect, for electrical instruments delicate 

 enough to detect a small fraction of the normal force, give no indi- 

 cations of the approach of a thunderstorm, and it is only when the 

 cloud has begun to discharge rain or hail that strong electrical effects 

 are noticed. During the thunderstorm the electroscope is, of course, 

 much disturbed, and there are frequent and violent reversals of its 

 indications.* The fact that no effects are observed at the surface of 

 the earth during the approach of a thunder-cloud does not prove that 

 there is no electrical separation, for we may imagine two oppositely 

 electrified layers at different levels producing a strong electric field 

 between them, but only weak effects outside. That some such thing 

 may possibly occur is indicated by observations made in mountain 

 districts, where violent electrical disturbances are observed previous 

 to the formation of clouds.f The cumulus cloud, from which the 

 lightning strikes out, is nearly always associated with a cirrus layer 

 above it, and the flash occurs more frequently upwards or sidewards 

 between the clouds than down to earth. Under such circumstances 

 it is clear that instruments on the surface of the earth can only very 

 partially indicate the nature and distribution of electrical stress in the 

 neighbourhood of the cloud. 



Thunderstorms seem always to be connected with a vortex motion, 

 and meteorologists distinguish two kinds of thunderstorms. The first 

 kind forms in the outlying portions of a large cyclonic system. The 

 storms which occur in winter are mostly of this nature, and the vortex 

 necessary for its formation is of the nature of a secondary disturbance. 

 The thunderstorm which forms in summer, on the other hand, makes 

 its own vortex, and is of a much more local character than that which 

 is produced round a previously-established barometric depression. 

 The summer storm is much influenced by the character of a district. 

 There are certain configurations apparently favourable to its forma- 

 tion, as is clearly brought out by the charts which have been made 

 representing their frequency. 



The route travelled over by the storm is affected by mountain 

 ridges, and rivers also seem to offer a peculiar impediment. Many 

 of them are brought to an end either along their whole front, or only 

 part of it, when they reach the banks of a large river.J 



Some curious problems are presented by the detailed structure of 

 lightning flashes. Although these lie outside the range of the present 

 lecture, reference must be made to the very beautiful photographs of 

 lightning flashes taken both in this country and abroad. The ordinary 

 forms which lightning takes are familiar to all ; but a good deal of 

 mystery still surrounds the so-called globular lightning. The manner 



* Weber, ' Elektroteclmische Zeitschrift,' vol. x. ; Elster and Geitel, ' Ueber 

 einige Ziele und Methoden Luftelektrischer Untersuchungen,' Wolfenbiittel, 

 1891. f Trabert, ' Meteorologische Zeitschrift,' 1889, p. 342. 



X Bebber, ' Meteorologie,' p. 255 ; Bornstein, ' Archiv der See wart,' viif .. 

 1885. 



Vol. XIV. (No. 89.) 2 k 



