VARIATION (continued)

The high reputation of Dr. Halley's magnetical chart renders it more particularly necessary to point out the errors contained therein (Euler. De la Lande). There is evidently too little distance betweeen the lines of no variation, of which one crosses the equator 17� westward of London, and the other 19� 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 further distant from the magnetic pole towards the south than in the north, as is required by the other phaenomena. 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 6o� 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, 51/2 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� 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-fix years appears to be 17� 1/4 W. and the least 16 �1/2 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� 1/4 W.; and in latitude 10� S. longitude 5� W. from London, from 2� 1/4 W. to 12� 3/4 W.; and in latitude 15� N. longitude 20�, it has increascd from 1� W. to 9� W.

But there is still a more extraordinary appearance in the Indian seas. For instance, under the equator: Euler.

Reference Works