The newe attractiue Containyng a short discourse of the magnes or lodestone, and amongest other his vertues, of a newe discouered secret and subtill propertie, concerning the declinyng of the needle, touched therewith under the plaine of the horizon. Now first founde out by Robert Norman hydrographer. Heerevnto are annexed certaine necessarie rules for the art of nauigation by the same R.N.

¶ Of the Variation of the Cum∣pas or Magneticall Needle. (Book 1)

Chapter I. (Book 1)

THE Variation of the Needle or Cumpas, is properly the ark of the Horizō contained betwéen the true meridiā of any place and the mag∣neticall meridian of the same, and is denominated to bee Esterly or Westerly, accordyng to the posi∣tion of the magneticall meridian to the Estwards or Westwards of the true meridian: And may be accompted either from the North parte, or the South parte thereof, but vppon opposit points it hath contrary denominations.

The magneticall meridian is to bee vnderstood a greate circle passyng by the Zenith and the Pole of the Magnes, deui∣dyng the Horizon into two equall parts crossyng the same at opposit points: which intersections or crossynges, are shewed by the Needle or wiers of the Cumpas touched with the Ma∣gnes or the Lodestone.

The Azimuth of the Sun is a great circle, passyng by the Zenith and the true place of the Sun: crossyng the Horizon at right Angles in opposit poincts, and diuidyng the same into two equall parts. And it is said to be giuen when the distance thereof from the true meridian is knowne.

The Azimuths of the Sun vpon equall eleuations in for∣noone and afternoone, haue equall distances from the true meridian, so that the middle poinct of the whole difference of any two Azimuths obserued vppon equall eleuations in for∣noone and afternoone, is the true meridian.

This difference of Azimuths is found vppon the Instru∣ment of Uariation, by addyng together the Uariations of the Sunnes shadowe at equall eleuations in fornoone and after∣noone. The half whereof is the distance of the Azimuths from the true Meridian: the whiche compared with either of the same variations of the Sunnes shadowe, the difference shalbe the variation of the Needle from the true meridian.

Or els subtracting the lesser variation of the Sunnes sha∣dowe, from the greater (at equall eleuations) the halfe of the remayner shall be the true variation of the Needle from the meridian.

But the Azimuth of the Sunne beyng otherwise giuen, and the variation of the shadowe likewise giuen, the difference betweene them is the variation of the Needle.

The Variation of the Sunnes shadowe I call, the Hori∣zontall distance betweene the Azimuth of the Sunne and the magneticall circle, whiche are represented in the Instrument by the shadowe of the line and the Needle.

[illustration]