The magnetic compass was invented some time during the first ten centuries AD. Credit is variously given to the Chinese, the Arabs, and the Italians. What is certain is that by the 12th century magnetic compasses were in regular use by mariners to aid navigation at sea. In the 13th century, Peter Perigrinus of France discovered that the magnetic effect of a spherical loadstone is strongest at two oppositely directed points on the surface of the sphere, which he termed the poles of the magnet. He found that there are two types of poles, and that like poles repel one another whereas unlike poles attract. In 1600, the English physician William Gilbert concluded, quite correctly, that the reason magnets like to align themselves in a North-South direction is that the Earth itself is a magnet. Furthermore, the Earth's magnetic poles are aligned, more or less, along its axis of rotation. This insight immediately gave rise to a fairly obvious nomenclature for the two different poles of a magnet: a magnetic north pole (N) has the same magnetic polarity as the geographic south pole of the Earth, and a magnetic south pole (S) has the same polarity as the geographic north pole of the Earth. Thus, the north pole of a magnet likes to point northwards towards the geographic north pole of the Earth (which is its magnetic south pole). Another British scientist, John Michell, discovered in 1750 that the attractive and repulsive forces between the poles of magnets vary inversely as the square of the distance of separation. Thus, the inverse square law for forces between magnets was actually discovered prior to that for forces between electric charges.