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A Treatise of Practical Surveying: Which is Demonstrated From its First Principles Wherein Every Thing That is Useful and Curious in that Art, is Fully Considered and Explained, 5th edition
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If the Variation is Westerly Decreasing or Easterly increasing.
Run as many Degrees to the left hand of the Course given as the difference is between the present and former variation, i.e. Subtract the Difference in the N.E. & S.W. which you must add to the N,W, & South, E. quarters of the Compass.
If the true meridian is not laid down on the old Map, it is customary to allow 1 Degree for 15 or 10 years & So in proportion for the number of years between the two Surveys. Surveyors run at present to the left of the Course given to fund the old lines which shows that the Variation is Westerly Decreasing; The Variation in Alexandria, m[?] 1791 was 30 minutes West.