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 
lass. 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 seclicconstructed 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 
the F was an element in the phenomena not sufficiently attended 
1ceC | beantiful illuminating effects by discharg- 
a wire | in an ed glass re- 
