53° 18´ The angle at the north pole, formed by the meridians passing through the two magnetic poles.
250° The longitude of the meridian, which passes over the northern magnetic pole.
As the observations which have been collected with regard to the variation are, for the most part, loose and inaccurate, it is impossible to represent them all with precision; and the great variations observed in the Indian ocean, seem to require, says M. Euler, that the three first quantities should be 14, 35, and 63 degrees. In the mean time, the general agreement is sufficiently satisfactory.
The high reputation of Dr. Halley’s magnetical chart renders it more particularly necessary to point out the errors contained therein[[56]]. There is evidently too little distance between the lines of no variation, of which one crosses the equator 17° westward of London, and the other 119° to the eastward. This makes 136 degrees only; whereas it should necessarily exceed 180 and even 200, inasmuch as the pole of the world is supposed farther distant from the magnetic pole towards the south than in the north, as is required by the other phænomena. Again, upon the coasts discovered by Diemen, there was no variation in 1642; and Dr. Halley also supposes there was none in 1700. Meanwhile, by the alteration observed at Paris, the line of no variation should be advanced 60° towards the south, which will agree better with the calculations, and prove that the distance of the two intersections was really greater than Dr. Halley had established.
The table of variation of Mess. Mountaine and Dodson is accompanied with several interesting particulars, which equally deserve to be inserted here.
At Barbadoes, (says Capt. Snow) the variation seems very nearly at a stand; for in the road I observed 5° east; and by Dr. Halley’s draught, in the year 1701, 5½ degrees. In 1747, at Port Royal keys, Jamaica, I observed the variation 7° 20´ E.; and on the coast of Carthagena, the same week, off the high land of Santa Martha, 7° 45´ nearly south of Port Royal. Therefore these curves are not much altered: the curve at Jamaica is nearly at a stand, as though tied, and the south part of them with the rest dropping to the westward.
Under the equator, in longitude 40° E. from London, the highest variation during the whole fifty-six years appears to be 17°¼ W. and the least 16°½ W.: and in latitude 15° N. longitude 60° W. from London, the variation has been constantly 5° E.: but in other places the case has been widely different. For in the latitude of 10° S. longitude 60° E. from London, the variation has decreased from 17° W. to 7°¼ W.; and in latitude 10° S. longitude 5° W. from London, from 2°¼ W. to 12°¾ W.; and in latitude 15° N. longitude 20°, it has increased from 1° W. to 9° W.
But there is still a more extraordinary appearance in the Indian seas. For instance, under the equator:
| LONGITUDE | MAGNETICAL VARIATION | |||
|---|---|---|---|---|
| East from London. | in 1700. | in 1756. | ||
| Degrees. | Degrees. | Degrees. | ||
| 40 | 16¾ | West. | 16¼ | West. |
| 45 | 17¾ | W. | 14½ | W. |
| 50 | 17½ | W. | 11¾ | W. |
| 55 | 16½ | W. | 8¾ | W. |
| 60 | 15¼ | W. | 6 | W. |
| 65 | 13½ | W. | 4½ | W. |
| 70 | 11½ | W. | 3¼ | W. |
| 75 | 9¾ | W. | 1 | W. |
| 80 | 7¾ | W. | 0¼ | East. |
| 85 | 5½ | W. | 1¼ | E. |
| 90 | 4¼ | W. | 1 | E. |
| 95 | 3¼ | W. | 0½ | West. |
| 100 | 2½ | W. | 1 | W. |
Where the west variation, in the longitude 40° E. is the same in both the above years; and in 1700 the west variation seemed to be regularly decreasing from longitude 50° E. to the longitude 100° E.; but in 1756, we find the west variation decreasing so fast, that we have east variation in the longitude 80°, 85°, and 90° E.; and yet in the longitude 95° and 100° E. we have west variation again. Philosophical Transactions for the year 1757.