The long arm was moved round until the two sights fixed upon it were on with the sun.

The point where the other arm then cut the circle gave the altitude.

In conjunction with this instrument were used the tables of solar declination compiled by Regiomontanus, and covering the sun's declination between the years 1475 and 1566.
The compass in Columbus's day existed, so far as all essentials are concerned, as it exists to-day.

Although it lacked the refinements introduced by Lord Kelvin it was swung in double-cradles, and had the thirty-two points painted upon a card.

The discovery of the compass, and even of the lodestone, are things wrapt in obscurity; but the lodestone had been known since at least the eleventh century, and the compass certainly since the thirteenth.