696 Proceedings of Royal Society of Edinburgh. [sess. 
ball is exposed to view in this form than if they were built up 
in the form of a cube or cylinder. Pointed bodies, then, must 
be looked upon as pyramids or cones of atoms ; and if the point 
be as sharp as possible, it will consist of but one molecule, or, 
possibly, atom, if the molecule to which it belongs happens to 
be pointing in the right direction. 
22. The Earth a Reservoir of Electricity. 
I think the reason for this will be readily seen. If any 
body be at a higher potential than the earth, and at the same 
time connected with it by a good conductor, the excess of motion 
in the body will escape into the earth, just for the same reason 
that steam would escape out of a boiler if a door were opened to 
it, or that water at a high level, or indeed anything else, con- 
stantly seeks to reach a lower. If the body be at a lower 
potential than the earth, or negatively charged, and connected 
with it by a good conductor, then motion would escape out of 
the earth to the body so as to bring it up to the same level as 
the earth, or to the neutral state — the earth in the above case 
illustrating the boiler. Positively charged bodies may be likened 
to water at a high level, the great neutral earth to the ocean, 
and negatively charged bodies to pits, or mines, under sea level. 
If the water from the high level were permitted to run into the 
pits it would fill them up to sea level — analogous to connecting 
two equally but oppositely charged bodies together. If the water 
from the high level Were permitted to fall into the sea — this 
would be analogous to connecting a positively charged body to 
earth, and if water from the sea were permitted to run into the 
pits it would be analogous to connecting a negative to earth. I 
think I need not elaborate this point any further, it seems 
perfectly obvious. 
23. By a proper application of the foregoing principles the 
results of all the experiments in electrostatics can be explained. 
For instance, take that of an insulated conducting cylinder under 
the influence of an electrified body. If the body be negative, 
the end of the cylinder under induction will become positive and 
the other end negative, because there is less pressure on the end 
