Page:First Voyage Round the World.djvu/250

 altitude of the sun. Turn the compass, until the thread which traverses it coincides with the direction of the meridian shade; then, from the antarctic pole of the needle, with the thread which remained over, draw another thread to the arctic pole, that is, to the flower; and you will thus find how many degrees the needle of the compass is distant from the meridian line, that is, from the true pole. So many will be the degrees of longitude, which you will have from the place where the compass begins to set itself in motion. Therefore, with the more accuracy you take the true meridian so much the more exactly will you be able to ascertain the degrees of longitude. And from this it may be seen that the meridian should never be taken with the compass, because it north-easts or north-wests, as soon as it goes out of the true meridian; but take an observation of the south with the astrolabe, and judge that it is midday when the sun is at its greatest height.

"If it is not possible to take the sun's altitude at midday, that can be determined with an hour-glass of sand, taking the hours of the night from sunset till the moment of its rising. Having learned the hours of the night, you will know how many are wanting of the twenty-four, and these you will divide into two equal parts. When half of this has elapsed, be sure that it is midday, and that the shadow indicates to you the true meridian. But since the sand clock may often be inexact, it will be better to take the sun's altitude with the astrolabe by means of its mediclino.