Magnetism

The earth is magnetic. Another happy occurrence, otherwise a compass

for instance would be a completely useless instrument. On a local scale

the magnetic field of the earth can vary, for example where large geo-

logical structures exist in the soil, or for that matter even a large iron bar.

 

Changes in the magnetism on the surface of the earth tell us that there is something

present in the soil. Interesting stuff when trying to trace a lost underground tank,

for example.