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"If you wish to navigate to any place, you must first know its position; that is, its latitude and longitude. Then, by means of the compass you will point directly to that place. And since the compass varies to east and west, you must, by the methods above described, ascertain its variation, and subtract or add that which is necessary, so that the ship's head, regulated by the compass, may have the required direction.

"Should the compass be lost, or if its variation east or west is not known, you may regulate yourself by the sun at midday. When you have fixed the meridian in such a manner that it cuts the ship in its width, it will be easy to direct the prow wherever you wish. Here is an example: suppose you wish to go from north-east to south-west, place the chart in such manner that the ship should have her head to west and the poop to the east; then on the circle of the winds, divided into 360, or in four times 90, fix two pins, one at 45 degrees between east and north, the other at 45 degrees between west and south; bring the two pins on the line of the meridian by turning the ship's head for that purpose, and the prow will be directed to the place to which you are going. If the pins do not come in a line with the meridian, it is a sign that you are navigating in a false direction, and you must rectify the course. When you reach land, you will see that what I have said is true.

"With an astrolabe made with plates, observations may be taken of the meridian line, the poles, and the equinoctial