Instrument is made of Brass, Ivory, Wood, or any other solid Matter, from 2 to 6 Inches in Diameter, being in figure of a Parallelopipedon, in the Middle of which is a round Box, at the Bottom of which is described a Card (of which more in the Construction of the Sea-Compass) whose Circumference is divided into 360 Degrees. In the Center of this Card is fixed a well-pointed Brass or Steel Pivot, whose Use is to carry the touched Needle placed upon it, in Equilibrio, so that it may freely turn. This Box is covered with a round Glass, for hindring left the Air should any wise agitate the Needle.
One of the Ends of the Needle always turns towards the North Part of the World, but not exactly, it declining therefrom, and the other towards the South.
According to Observations made in October, in the Year 1715, in the Royal Observatory, the Needle declined 2 Deg. 5 Min. Westwardly.
Needles are made of Pieces of Steel, the Length of the Diameter of the Box, having little Brass Caps soldered to their Middle, hollowed into a conical Figure so, that the Needle being put upon the Pivot, may move very freely upon it, and not fall off; they are nicely filed into different Figures, those which are large being like a Dart, and small ones have Rings towards one End, for knowing that End which respects the North, as may be seen in the little Figures nigh the Compass.
To touch a Needle well, having first got a good Stone, begin your Touch near the Middle of the Needle, and pressing it pretty hard upon the Pole of the Stone, draw it slowly along to the End of the Needle, and lifting your Hand a good Distance from the Stone, while you put the Needle forward again, begin a second Touch in the same manner, and after that a third, which is enough, only take Care not to rub the Needle to and fro on the Stone, whereby the backward Rubs take away what Virtue the forward ones gave; but lift it out of the Sphere of the Stone’s Virtue, when you carry it forward again to begin a new Touch.
This admirable Property, by help of which great Sea-Voyages were first undertaken, and vast Nations both in the East and West discovered, was not known in Europe ’till about the Year 1260.
A Man by means of this Instrument, and a Map, may likewise go to any proposed Place, at Land, without enquiring of any body the way; for he need but set the Center of the Compass, upon the Place of Departure, on the Map, and afterwards cause the Needle to agree with the Meridian of this Place upon the Map: then if he notes the Angle that the Line leading to the Place makes with the Meridian, he need but in travelling keep that Angle with the Meridian, and that will direct him to the Place desired.
This Instrument is also very useful to People working in Quarries, and Mines under Ground; for having noted upon the Ground the Point directly over that you have a mind to go to, you must place the Compass at the Entrance into the Quarry or Mine, and observe the Angle made by the Needle with the Line of Direction: then when you are under Ground, you must make a Trench, making an Angle with the Needle equal to the aforesaid Angle; by means of which you may come to the proposed Place under Ground. There are several other Uses of this Instrument, the principal of which we are now going to speak.