486 Mr. C. Tomlinson on the Weathering of Rocks, 



The light was seen by the sailors in the harbour coming in 

 from the sea and passing up the valley like a low cloud."" 



A very good observer, M. Trecul, relates the following cases *. 

 During a storm, on the 19th of August, 1880, sparks appeared 

 to proceed from some lightning-conductors, some rising singly 

 and disappearing at a small height, expanding into a circular 

 flash, the light of which diminished from the centre to the 

 circumference. In one case two luminous columns rose 

 simultaneously and parallel, and at a certain height coalesced 

 at right angles. On August 25th, in the same year, during 

 a thunder-storm and in full daylight, he saw a very brilliant 

 body, slightly elongated (say from 30 to 40 centimetres long 

 by 25 cm. broad) and with conical ends, pass from one part 

 of a dense cloud to another. He states that he has often 

 noticed during thunderstorms a band of feeble light moment- 

 arily illuminating a street, and reaching right across or only 

 part of the width. 



M. Trecul also relates the following curious case : — On the 

 18th of August, 1876, while writing at an open window, 

 between 7 and 8 a.m., he observed, simultaneously with some 

 loud thunder, small luminous columns descend obliquely on 

 his paper, about two metres long and half a decimetre broad 

 at the widest part ; obtuse at the farther end, but gradually 

 thinning towards the table. They had mostly a reddish-yel- 

 low tint : near the paper the tints were more intense and 

 varied. In disappearing they left the paper with a slight 

 noise like that produced by pouring a little water on a hot 

 plate. 



Cases of this sort might be greatly multiplied, as also of 

 lightning quitting the conductor. The earliest case of this 

 kind occurs in Franklin's works (vol. i. p. 340), in which an 

 eye-witness relates that " he saw the lightning diffused over the 

 pavement, which was then very wet with rain, to the distance 

 of two or three yards from the foot of the conductor." 

 Franklin very properly remarks on this statement, that " it 

 seems to indicate that the earth under the pavement was very 

 dry, and that the rod should have been sunk deeper till it 

 came to moister earth, and therefore apter to receive and dis- 

 sipate the electric fluid " (p. 358). He also gives a curious 

 case in which the lightning left the conductor on the outside 

 of a wall to strike the more capacious barrel of a gun resting 

 against the inside of the wall. 



I conclude with a few remarks on African thunderstorms, 

 in addition to what has already been said on that subject. 



* Comptes Rendus, 1880-81. 



