118 Experimental Philosophy. [Lecture 9. 



tending its branches over a large tract of country, 

 the lightning is seen to dart from one part of it 

 to another, and often to illuminate its whole 

 mass. When the cloud has acquired sufficient 

 extent, the lightning strikes between it and the 

 earth in two opposite places. As the lightning con- 

 tinues, the cloud dilates, till at length it breaks 

 in different places, and displays a clear sky. 



The clouds, however, are sometimes nega- 

 tively electrified with respect to the earth, and in 

 this case the lightning is supposed to proceed 

 from the earth to the cloud ; but the mischievous 

 effects are the same, and, in fact, there is reason 

 to think that this is a rare case. 



During a thunder storm the safest place is in a 

 cellar; for when a person is below the surface 

 of the earth, the lightning must strike it before it 

 can reach him, and its force will therefore pro- 

 bably be expended on it. When it is not possi- 

 ble to retreat to a cellar, the best situation is in 

 the middle of a room, not under a metal chande- 

 lier, or any other conducting surface ; and it is 

 adviseable to sit on one chair, and to lay the feet 

 up on another; or it would be still better to 

 lay two or three beds or mattresses, one upon 

 another, in the middle of the room, and place 

 the chairs upon them, the matters (viz. hair and 

 feathers) with which they are stuffed being non- 

 conductors. Persons in fields should prefer the 

 open parts to any shelter under the trees, &c. 

 The distance of a thunder cloud, and conse- 



