26 Professor Leslie on Electrical Theories. 
scend with that slow motion which is sufficient to occasion a re- 
sistance on their large surface, equal to their gravitation. If 
the cloud * thus generated, reach the ground, it will soon com- 
municate its electricity. If it be suspended at some height, the 
electrified air will stream from it in all directions; and if its for- 
mation be gradual, this discharge may suffice to waste its 
force. But when a vast cloud is suddenly formed, the aerial 
emission hardly impairs its electricity ; and as it is carried along, 
it continually approaches, by its attraction, to the surface, 
which assumes an opposite electricity ; the air now rushes with 
violence, and the cloud bends faster downwards, till at last its 
lowest verge reaches the ground, and a total discharge is made. 
The magnitude of the stroke will evidently depend on the ex- 
tent of the aqueous mass, the suddenness of its precipitation, 
and the rapidity of its descent. 
The air, which streams in all directions from the cloud, is dis- 
sipated among the more remote portions, and thus gradually com- 
municates its electricity. Hence, from the wide dispersion, ow- 
ing to the distance, the electricity of the air at the surface of 
the earth must be weak ; and, even in the midst of the storm, the 
electrometer is less affected than if placed only a yard behind the 
prime conductor. Yet the action of the thunder-rod is confined 
entirely to the air which immediately surrounds it, and the 
quantity of aerial current which it can produce, must evidently 
be inferior to what is directed to the point, w r hen held several 
feet from the conductor of an electrical machine. But to avert 
the stroke, it would be necessary that the whole air between the 
surface and the cloud should be brought successively in contact 
with the top of the rod. Nor is this all ; for the air will be con- 
stantly replaced by other electrified portions emitted from the 
cloud. The effect of the thunder-rod is therefore, comparatively, 
but a drop in the ocean -J-. It may be easily shewn, that, hcnvever 
pointed and tapered, it would require a thousand years to guard 
at the distance of an hundred yards ; if terminated with a knob, 
a It is a common observation in the country, that, after a thunder-cloud has 
passed, the wind still blows from the cloud, though in an opposite direction. 
•f It appears, from the experiment with the heated ball, that a good kitchen- 
fire has more efficacy in preventing a house from being struck, than a whole ma- 
gazine of thunder-rods. Hence one of the reasons why a thunder-cloud diminishes 
so fast in passing over a large city. 
