TRANSACTIONS OF SECTION A. 507 



8. Notes on Almospheric JEledricity and the use of Sir William Thomson's 

 portable Electrometer in the Tropics. By Profossor C. MicuiE Smith. 



Recent observations fully con6rm the author's previous conclusions, that in 

 Madras, with a dry west wiud, the potential of the air for some distance 

 above tlie ground is usually negative. Tliis seems always to be associated with 

 dusty air. The potential of the air remains positive till between 9 and 10 a.m., then 

 becomes negative, and continues so till the sea breeze sets in. Electrometer 

 observations on evenings when sheet lightning is seen point to the conclusion that, 

 while much so-called sheet lightning is reallj' simply tho reflection of distant flashes,^ 

 it is more often due to discharge taking place between two neighbouring clouds. 

 In this case the electrometer is unaifectud, whereas it is strongly influenced by the 

 discharges of a distant storm. This conclusion is .strongly confirmed by other 

 obsei'vations. Two series of observations made in Japan were tlien described ; one 

 made during the a.scent and descent of the active volcano of Asamayama, the other in 

 crossing the Wada I'ass. 



In using the portable electrometer in hot moist climates, special precautions 

 have to be taken in drying it. As much sulphuric acid must be used as the pumice 

 can absorb, and the pumice should be dried at least once a fortnight. The charging 

 rod itself must be very carefully dried, and after charging should be lifted out with 

 a piece of warm silk. 



9. On Photographs of Lightning. By Professor Leonhard Weber. 



Two photographs obtained with an oscillating camera were exhibited; the 

 peculiarities of the pictures were described, and conclusions were drawn from them. 



10. On Bark Flashes of Lightning. By A. W. Clayden, M.A. 



The author exhibited a negative taken June Gth, 1889, which shows several 

 reversed images of lightning flashes. lie described a series of experiments which 

 he had carried out with the object of discovering whether the phenomenon could be 

 imitated in the laboratory. The steps in the investigation were illustrated by a 

 series of negatives showing photographs of electric sparks. 



The conclusions arrived at are that photographic images of electric sparks can 

 be reversed by the action of diifused light, lleversal is only produced when the 

 exposure to ditfused light is subsequent to (or possibly simultaneous with) exposure 

 to the image of the spark. If the plate is first acted on bj' ditfused light, the sparks 

 give a direct image unless the action has been considerable, in which case they 

 seem to make no impression. 



If the plate is exposed to sparks in one direction, then to diflused light, and 

 again to sparks in some other direction, the first set of sparks show reversal, while 

 the images of the second set will be direct. This explains the crossing of a ' dark 

 flash ' by a bright one. 



When the image of the spark is dense, reversal begins on the margin and works 

 inwards towards the denser core, and the more brilliant the sparks the greater the ex- 

 posure to diffused light required for reversal. This, in turn, explains the reversal of 

 comparatively faint branches of a flash, while the principal line of discharge 

 gives a direct image. 



The reversing-light may be either gas-light, that from a paraffin lamp, lime 

 light, magnesium-light, daylight, or even the diffused light of electric sparks. In 

 the last case, reversal is more easily produced if the light passes through ground 

 glass, or is reflected from a white surface, than if it is allowed to fail directly on 

 the plate. 



Iho explanation of these photographic phenomena is obscure. The intensity of 

 the light does not seem to have anytliing to do with the matter, except that the 

 effects of the spark and reversing light must be about equal. Nor does the refrangi- 

 bility of the reversing light oiler an explanation, for the reversing power is appa- 

 rently proportional to the actinic power. 



The author is pursuing the research. 



