18 British Association for the Advancement of Science. 



Some remarks were then made on the theory of thunder-storms, 

 as given by the French writers, in which the cloud is considered 

 as analogous in action to one coating of a charged glass, the earth 

 the other coating, and the air between as the non-conducting 

 glass. One very material circumstance has been overlooked in 

 this theory, — namely, the great thickness of the intervening stra- 

 tum, and the consequent great quantity of free or redundant elec- 

 tricity in the cloud. This must modify the nature of the dis- 

 charge from the thunder-cloud, and lead to doubt, if it be per- 

 fectly analogous to the discharge from an ordinary Leyden jar, 

 since the great quantity of redundant electricity must produce a 

 comparatively greater lateral action ; and hence, possibly, the ram- 

 ifications of the flash, and other similar phenomena, may be but 

 cases of the lateral discharge. 



Some facts were then mentioned, on the phenomena of the 

 spark from a long wire charged with common or atmospheric 

 electricity. It is well known that the spark in this case is very 

 pungent, resembling a shock from a Leyden jar. The effect does 

 not appear to be produced, as is generally supposed, by the high 

 intensity of the electricity at the ends of the wire by mere distri- 

 bution, since this is incompatible with the shortness of the spark. 

 In one experiment, fifteen persons joining hands received a severe 

 shock, while standing on the grass, from a long wire, one of the 

 number only touched the conductor ; the spark in this case was 

 not more than a quarter of an inch long. 



Mr. Sturgeon was confident a well-constructed thunder rod 

 would never be struck by lightning, as, upon the approach of an 

 electrical cloud, it would silently discharge it into the earth. — 

 Mr. Stevelly said, that unquestionably when the discharge was 

 made directly upon the thunder rod, if well constructed, it would 

 perform its office silently ; but if a lateral discharge took place 

 near it, the effect, as Prof. Henry showed, might be flashes of 

 light and heat from the entire length of it, capable, when on a 

 great scale, of setting fire to buildings, firing gunpowder, and 

 other effects hitherto unexpected. — Mr. Snow Harris expressed 

 his regret, that he had not been in the room during the early part 

 of Prof. Henry's communication. In his opinion, the pressure of 

 the air was an element in the phenomena not sufficiently attended 

 to. He had produced beautiful illuminating effects by discharg- 

 ing electricity along a wire enclosed in an exhausted glass re- 



