Sound travels about thirteen miles a minute; so that if we hear a clap of thun¬ 
der half a minute after the flash we may calculate that the rge of electricity 
* . 1 fired at a dis- 
s heard. 
)ver a space of 
lould maintain 
y, and yet light 
Or 
key, has been 
it has a grayish 
and sulphide 
