He remarks upon the 'singular independence' of the magnetism and the body of the magnet which carries it.

The steel behaves as if it were isolated from its own magnetism.
And then his thoughts suddenly widen, and he asks himself whether the rotating earth does not generate induced currents as it turns round its axis from west to east.

In his experiment with the twirling magnet the galvanometer wire remained at rest; one portion of the circuit was in motion relatively to another portion.

But in the case of the twirling planet the galvanometer wire would necessarily be carried along with the earth; there would be no relative motion.

What must be the consequence?
Take the case of a telegraph wire with its two terminal plates dipped into the earth, and suppose the wire to lie in the magnetic meridian.
The ground underneath the wire is influenced like the wire itself by the earth's rotation; if a current from south to north be generated in the wire, a similar current from south to north would be generated in the earth under the wire; these currents would run against the same terminal plate, and thus neutralise each other.
This inference appears inevitable, but his profound vision perceived its possible invalidity.