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Observations should be constantly taken at noon. The same Vari-
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ation will not be found at other hours of the Day (: this is no reason
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variation needle is placed in an observatory, and that that observ-
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atory is only, like his, visited about Noon.
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As the magnetic Line does not run truely through the points of a
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magnetical needle, on both the surfaces of the needle: the surfaces
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of this proposition is false and the other badly expressed. the mag-
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netic line may run through the axis of the needle, and there is no
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doubt but needles might be made, whose axis would coincide with
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the magnetic line. Further to say that the magnetic line does not run
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through the same points on both surfaces of the needle is downright
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same point the two points | 10v = 2opposite one another, or corresponding
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one another in both the surfaces of the needle, considering the
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faces. He meant to say, that the magnetic line does commonly make
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an angle with the axis of the needle which is found by turning the
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needle, and putting the upper side under most, just as the line of
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collimation in quadrant is rectified by observing the same star near
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the Zenith once with the Limb to the East and another time with the
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same to the West.
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When the Variations of both surfaces are taken, add their sums to-
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gether and the half of such an addition gives the mean Variation. (It is
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not the mean variation but the true one, which is the mean between
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Thus when the compasses were made for the Royal Observatory at
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Richmond
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I found on the 17 of December 1770 that one side
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of the needle gave20°36West
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the other2152–
at Noon
at Noon
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Sum4228
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mean Variation2114West