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IV. Propositions assistent to the finding of the Variation of the Compass.
THE Propositions that will be serviceable herein are the 3. 7. 8. 9. 10. 11. and 14. but more especially the Third and the Eleventh: and those I shall here illustrate by Exam∣ple, though all the rest (as occasion may fall out) will be also usefull thereunto. By the Third Proposition you may find the Amplitude of the Sun's Rising and Setting.—By the Eleventh you may find the Azimuth at any time of the day.— By either of which the Variation of the Compass may be found, and also which way it varieth.
BY the Third Proposition you found the Sun's Amplitude at his rising or setting to be 33 degr. 20 min. from the true East or West Points of the Horizon towards the North. Having thus before-hand found the Amplitude, in the Morning I set my Compass to the Sun at his Rising; and if I find that the Sun by my Compass do rise 33 d. 20 m. from the West-point thereof towards the North, then may I be ascertain'd that my Compass hath no Variation, but that the Fly or Wires do point directly North and South.—But finding before-hand the Amplitude to be 33 d. 20 m. and I should find the Sun to rise but 28 degr. from the East-point of my Compass, then substracting 28 degr. from 33 degr. 20 min. the difference is 5 degr. 20 min. and so much doth my Compass varie from the true East-point, and con∣sequently all the other Points of the Compass as much.
Now to find which way the Compass varieth, you must ob∣serve whether your Amplitude, found by your Calculation, be to the Right or Left-hand of the Sun's rising or setting. And