Academia scientiarum, or, The academy of sciences being a short and easie introduction to the knowledge of the liberal arts and sciences, with the names of those famous authors that have written on every particular science : in English and Latine / by D. Abercromby ...

Sect. XXII. The Art of Sailing.

ARS Nautica, or Histiodromica, is that Art which teacheth how to direct a Ship through the Seas, to the propos'd Harbour.

This Art requireth the knowledge of the Mariners Compass, and the Lead, of the Sea-coasts, Capes, Rocks, Promonto∣ries, Harbours, of the distances of one place from another, of the ebbing and flowing of the Sea, of the latitude and longitude of every place. It requireth likewise the knowledge of several Instru∣ments fit to take the latitude of a place, as of the Cross-staff, of the Quadrant, of the Nocturnal, of the Plane Scale, of Gunter's Scale, &c.

The Mariners Compass is a round Plane, whose circumference is divided into 32 equal parts, by streight lines, called Rhombs, passing through the center.

The height of the Pole, of so great benefit to Sailers, is found out thus: Observe first the height of the Sun at Noon-day, with an Astrolabe, or some other Instrument of that kind; then take the declination of the Sun, from the height, if the Sun declines from the E∣quator towards the Northern Pole; or add the declination of the Sun, to the ob∣served height, if the Sun declines to∣wards the Southern Pole; the remaining number, or the sum made up by Additi∣on, gives you the height of the Equator, whose Complement to 90 degrees (as they speak) is always the height of the Pole. Thus if the height of the Equator above our Horizon be 60 deg. the height of the Pole is 30 deg. because 30 added to 60, make up 90; and if the Pole be eleva∣ted but 10 deg. the height of the Equator is 80, because this number is the Com∣plement of that.

If their could be an Hour Glass, or •• Clock, so contriv'd, as to fall but very little short of the Measure of Time▪

with the help of this Clock, to the great advantage of Sailers, the differences of the longitudes might be found after this manner: When the Ship sets off, let the Clock shew the hour in the place from whence you sail'd, without discontinu∣ing: If then we would know the longi∣tude of the place in which we now are, let us, by observation of the Sun, find the hour in that place we chance to be in; which if it be the same pointed at by the Clock, or shewn by the Glass, 'tis cer∣tain we are in the same Meridian we were in at our first setting out; but if we find by observation, more hours than the Clock pointeth at, we have made a progress towards the East; if we find fewer hours, we are gone towards the West; and the differences of the longi∣tudes may easily be known, if the diffe∣rences of the hours be converted into de∣grees, and minutes of degrees.

Authors.

Seller, Everard, Wright, &c.