Page:Philosophical Transactions - Volume 002.djvu/36

 The same may be done with an ordinary Sea Compass, so it have a Circle towards the Limb of the Card, divided into Degrees, by fastening a small Thread, Lute-string or Wire (not of Iron) so upon it, as to pass just over the Center of that Circle; and placing a strait piece of wood or Brass-wire perpendicular on the edge of the Box at the end of the Thread, and turning it to the Sun, till the Shadow of it fall just upon the Thread: then observe, what Degree of the Circle on the Card the Thread cuts, by looking plum upon it; and that is the Suns Magnetical Azimuth.

But to have the Variation to Degrees and Minutes (which is most definable) then the Observation last mention'd must be made with a Quadrant, Sextant, or some such other Instrument, so large as to admit of the division of a Degree into Minutes; which will require the Radius to be about three foot; the larger the better. If a Quadrant, then, it being laid flat, and the Square Box with the Needle placed upon it, move the Quadrant to and again, till that side of it, on which the Box is placed, lie parallel to the Needle, when at quiet: Then the Sight of the Quadrant being slid along the Rimb of it, till the Sun shine on both its sides at the same time, the Mid-Line, that divides equally the Sight, when the Sun shines upon it through the slit, will mark the Degree. and Minute of the Suns Magnetical Azimuth. All which is easie to be put in practice.

And if many such Observations be made by Several persons at the same place; and by the same or other persons distant from one another, 1.10.20. or more Years; Not only will the Compass become more useful then formerly, even to be conducive possibly to the finding the Lougitude at Sea, at least in some places: but the variation of the Variation of the Needle being known in different places, all will be reduced to Rules, and so from hence, Philosophical or Natural Knowledg, will probably be enlarged by a happy discovery of the true cause of the Verticity, or Directive faculty of the Loadstone; one of the Noblest and most abstruce Phænomena, that falls under the cognizance of humane Reason.

To find this variation by the Stars, is so easie, as every Master can do it; seeing there is no more requisite, than to find out the true North, that is the Meridian, and compare the Needles on