Direction of the Ship.
“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,[433] observations may be taken of the meridian line, the poles, and the equinoctial line, at any hour of the day or night, looking at the moon or any star; and for these, place in the middle of the astrolabe instead of the verghezita or sight,[434] two straight bars between which you will observe the star.”
Thus the method being known by which the required direction is given to the ship, the author teaches the method for determining the point or degree on the chart of the winds,[435] to which the ship on leaving a place to go to a given country should be directed. For clearness, he gives some examples of this. “Do you wish,” he says, “to go from south to north, or vice versâ, on the same longitude? always proceed on the same meridian. Do you wish to go from east to west, or vice versâ, in the same latitude? always proceed on the same parallel. Do you wish to go from one place to another as many degrees distant in longitude as it is different in latitude? Then take the course of 45 degrees either to the south-west or south-east, or north-west or north-east. If the latitude is greater than the longitude, then add to the 45 degrees as many degrees towards the nearest pole, as the number of degrees by which the latitude exceeds the longitude. For instance, if I wish to go from Cape St. Vincent to Cape Bojador, I reckon the degrees of longitude and those of latitude to know the difference between these two capes. I find that the degrees of longitude are five and a half, and those of latitude are eleven, from which I subtract the degrees of longitude and there remain 5 deg. 30 min. Then, instead of going in the direction of from north-east to south-west (as I should do if the longitude were equal to the latitude), I go from 5 deg. 30 min. above north-east towards north, to 5 deg. 30 min. below south-west towards south. If the longitude is greater than the latitude, the lesser number is still subtracted from the greater; and the direction will be 45 deg. after deducting the residue. For instance, do you want to go from the island of Ferro to Guadeloupe; you know that the first is in 27 deg. latitude and the second in 15 deg.; then take their difference, which is 12 deg.: look at the map for their longitude, and you see that Ferro is in 1 deg. and Guadeloupe in 45 deg., whence their difference is 44 deg.: subtract from these the 12 deg. residue of latitude, and there remain 32 deg. Then you must subtract these 32 deg. from 45 deg., and there will remain 13 deg. Therefore your course will be from north-east 13 deg. north to south-west 13 deg. south.”