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To know whether the variation be easterly or westerly.
Just as with the amplitudes, let the observer's face be turned to the sun ; then if the true azimuth be to the right hand of the magnetical one, the variation is easterly; but if to the left, westerly.
In the latitude 53o. 20 N. the sun's declination being 19o. 03, N. I find by observation the sun's altitude to be 37o. 30', and its magnetical azimuth to be SE. 51o. Required the variation.
90°–53.20'=36.40, the compt. of the latitude
18.20. į the compt. of the latitude
90°—370.30'=52°30, the compt, of the altitude.
90°-19o.03'=70°.57', the sun's dist. from the pole
52.30 compt. of the altitude
61.43 half sum
As the tang. of į the compt. of the latitude,
Is to the tangent of į the sum of the distance of the sun from the pole and complement of the altitude,
:: tang. of their difference
To å tangent of a fourth arc. 42. 18–9.95907
Co-sine of the sun's true azim. 70.049.53288
S. 700.04' E.
19. 04 W.
The true azimuth being to the left of the magnetic one, the variation is westerly.
Suppose the sun's true azimuth N. 83°, 20' E. but the magnetical one N. 709.30' E. Required the variation.
N. 830.20 E.
12. 50 E.
The true azimath being to the right of the magnes tical one, the variation is easterly.
Suppose the sun's true azimuth was S. 370. 15W. and the magnetical one S. 44°.20W. Required the variation.
S. 370. 15' W.
7. 05 W.
The true azimuth being to the left of the magnetical one,
the variation is westerly.
Suppose the sun's true azimuth be S. 4°.05 W. and the magnetical one S. 30.30 E. Required the variation.
S. 4°.05' W.
The true azimuth being to the right of the magnetical, the variation is easterly.
The variation of the compass was first observed at. London in the year 1580, to be one point of the compass easterly, or 11°.15' E. after which time it became less; for in the year 1622 it was 69.00' E. in 1634 it was 4°05' E. and so continued to decrease till the needle coincided with the true meridian, and then there was no variation; after which the variation became westerly, and has ever since increased to the westward; for in the year 1672 it was 20.33' W. in the year 1683 it was 49.30' W. at London; in 1722 it was at Dublin found to 110.15' W'. and in 1751 it was there found to be 199.00' W. At London, in the year 1800, it was about 23°.30', and still continues to increase westerly at the rate of 11 or 12' every year,
At Paris in 1610, the variation was 30.00' E. in 1666 there was no variation ; but in 1681 it was 20.30' W. and still continues to go on westerly.
How to draw a true meridian line to a map, having
the variation and magnetical meridian given.
On any magnetical meridian or parallel, upon which your map is protracted, set off an angle from the north towards the east, equal to the degrees or quantity of variation, if it be westerly, or from the north towards the west if it be easterly, and the line which constitutes such an angle with the magnetical meridian, will be a true meridian line.
For if the variation be westerly, the magnetical me. ridian will be the quantity of variation of the west side of the true meridian, but if easterly on the east side, therefore the true meridian must be a like quantity on the east side of the magnetical one, when the variation is westerly, and on the west side when it is easterly,
How to lay out a true meridian line by the circum
If the variation be westerly, turn the box about till the north of the needle points as many degrees from the flower-de-luce towards the east of the box, or till the south of the needle points the like number of degrees from the south towards the west, as are the number of degrees contained in the variation, and the index will be then due north and south ; therefore, if a line be struck out in the direction thereof, it will be