Edinburgh, and cause the magic needle stationed there to move.
The deflections of this needle may be converted into intelligible signs. They can be made to spell words; thus, one movement may stand for a; two for b; three for c, and so on to the end of the alphabet.
We have said that our courier will refuse to leave the battery unless he be provided with a return ticket, or in other words, unless he can secure a safe passage home; it does not follow, however, that his homeward path must be a wire, as by a peculiar arrangement we can force him to find his way from Edinburgh to London through the earth.
We have supposed that only one kind of motion can be given to the magnetic needle, and that the Amber Spirit can only be made to travel in one direction, that is to say, from the copper end of the battery through the wire, and back again through the earth. If we connect the wire with the zinc end this direction is reversed, and, as a matter of course, the Spirit passes over the needle from north to south, instead of from south to north as before.
This new direction is at once detected by the needle, and its north pole moves to the right, whereas it had previously moved to the left. We may take advantage of this double movement in simplifying our alphabet; thus, one movement to the right may stand for a; one to the left for b; one right and one left for c, and so forth.