166 Things not generally Known. 



of distant thunderstorms, Saussure observed sheet-lightning in the di- 

 rection of Geneva, from the Hospice du Grimsel, on the 10th and llth 

 of July 1783 ; while at the same time a terrific thunderstorm raged at 

 Geneva. Howard, from Tottenham, near London, on July 31, 1813, 

 saw sheet-lightning towards the south-east, while the sky was bespan- 

 gled with stars, not a cloud floating in the air ; at the same time a thun- 

 derstorm raged at Hastings, and in France from Calais to Dunkirk. 

 Arago supports his opinion, that the phenomenon is reflected lightning, 

 by the following illustration : In 1803, when observations were being 

 made for determining the longitude, M. de Zach, on the Brocken, used 

 a few ounces of gunpowder as a signal, the flash of which was visible 

 from the Klenlenberg, sixty leagues off, although these mountains are 

 invisible from each other. 



PRODUCTION OF LIGHTNING BY RAIN. 



A sudden gust of rain is almost sure to succeed a violent 

 detonation immediately overhead. Mr. Birt, the meteorologist, 

 asks : Is this rain a cause or consequence of the electric discharge ? 

 To this he replies : 



In the sudden agglomeration of many minute and feebly electrified 

 globules into one rain-drop, the quantity of electricity is increased in a 

 greater proportion than the surface over which (according to the laws of 

 electric distribution) it is spread. By tension, therefore, it is increased, 

 and may attain the point when it is capable of separating from the drop 

 to seek the surface of the cloud, or of the newly-formed descending body 

 of rain, which, under such circumstances, may be regarded as a conduct- 

 ing medium. Arrived at this surface, the tension, for the same reason, 

 becomes enormous, and a flash escapes. This theory Mr. Birt has con- 

 firmed by observation of rain in thunderstorms. 



SERVICE OF LIGHTNING-CONDUCTORS. 



Sir David Brewster relates a remarkable instance of a tree 

 in Clandeboye Park, in a thick mass of wood, and not the tallest 

 of the group, being struck by lightning, which passed down the 

 trunk into the ground, rending the tree asunder. This shows 

 that an object may be struck by lightning in a locality where 

 there are numerous conducting points more elevated than 

 itself ; and at the same time proves that lightning cannot be 

 diverted from its course by lofty isolated conductors, but that 

 the protection of buildings from this species of meteor can only 

 be effected by conductors stretching out in all directions. 



Professor Silliman states, that lightning-rods cannot be re- 

 lied upon unless they reach the earth where it is permanently 

 wet ; and that the best security is afforded by carrying the rod, 

 or some good metallic conductor duly connected with it, to the 

 water in the well, or to some other water that never fails. The 

 professor's house, it seems, was struck ; but his lightning-rods 

 were not more than two or three inches in the ground, and were 

 therefore virtually of no avail in protecting the building. 



