From the first instant that we made this affair the object of our more particular consideration, we have attended to the mode of increase and decrease in the variation: and as a considerable number of observations, made at periodic times, and duly registered, seem to be the most essential toward determining the laws of its mutation, or proving its irregularity, we have therefore formed a sett of tables, from actual observations collected for the years 1710, 1720, 1730, and 1744, the date of our last chart; which, together with Dr. Halley's for the year 1700, and the present chart now publishing, compleat six reviews: These are tabulated, and shew the quantity of the variation, at those several periods, to every 5 degrees of latitude and longitude in the more frequented oceans; which we hope will prove acceptable, as nothing of the like kind has yet appeared, or can easily be obtained.

Our materials have been so deficient, that even in the limits to which our tables are confined, we have been obliged to leave blanks in some of the above periods, for want of that concurrent testimony, on which the numbers inserted are founded: but, considering the difficulties unavoidably attending a work of this sort, and the little assistance which we have met with from private hands, we hope that this Royal Society will not only excuse those vacancies, but also those in the great tracts of sea, as well as land, concerning which we are very unwillingly obliged to be intirely silent.

Agreeable to our former address, we lay only what appear to be facts before you, without attempting to introduce any hypothesis for the solution of these phænomena; some of which (being very extraordinary) we recommend peculiarly to the notice of those gentlemen, who may endeavour the investigation of their causes.

Under the equator, in longitude 40° E. from London, the highest variation during the whole 56 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 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, it has increased from 2°¼ W. to 12°¾ W.; and in latitude 15° N. longitude 20° W. 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 from London Variation in
1700. 1756.
Degrees. Degrees. Degrees.
40 E 16¾ W 16¾ W
45 E 17¾ W 14½ W
50 E 17½ W 11¾ W
55 E 16½ W 8¾ W
60 E 15¼ W 6 W
65 E 13½ W 4½ W
70 E11½ W2¾ W
75 E 9¾ W 1 W
80 E 7¾ W 0¼ E
85 E 5½ W 1¼ E
90 E 4¼ W 1 E
95 E 3¼ W 0½ W
100 E 2½ W1 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.

Such are the irregularities, that experience hath shewn us, in the variation of the magnetic needle; which appear so considerable, that we cannot think it wholly under the direction of one general and uniform law; but rather conclude, with the learned and judicious Dr. Gowen Knight, Fellow of this Society, in the 87th prop. of his treatise upon attraction and repulsion, That it is influenced by various and different magnetic attractions, in all probability occasioned by the heterogeneous compositions in the great magnet, the Earth.

Notwithstanding all which, should the sagacity of some eminent philosopher be able to exhibit rules, whereby the quantity of the variation may be computed for future times, yet then such a review, as we have now made, will be necessary at a proper interval, to prove the truth of them: and should no such rules appear, then will a continued succession of such reviews be necessary so long as commerce and navigation subsist among us.

What we have now done is intirely for the public service, the sale of the former chart never having made good its expence; and we propose to continue our endeavours for another review, at the proper time, if we shall then be alive, and capable of the task: but as the contrary may probably happen, we beg leave to conclude with recommending such a continuation, in the strongest manner, to such of the members of this Royal Society, or others, who may, at the proper intervals, have leisure and ability for such a performance.