150 PHILOSOPHICAL TRANSACTIONS. [aNNO 1757. 



From the first instant, say these gentlemen, that we made this affair the 

 object of our 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 raost essential to- 

 ward determining the laws of its mutation, or proving its irregularity, we have 

 therefore formed a set 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, 

 complete 6 reviews : These are tabulated, and show the quantity of the varia- 

 tion at those several periods, to every 5 degrees of latitude and longitude in the 

 more frequented oceans. Under the equator, in longitude 40° e. from London, 

 the highest variation during the whole 56 years appears to be 17° ± w. and the 

 least 16° -J- w. : and in latitude 15° n. longitude 6o° w. from London, the varia- 

 tion has been constantly 5° e. but in other places the case has been widely dif- 

 ferent ; for in the latitude 10° s. longitude 6o^ e. from London, the variation 

 has decreased from 17° w. to 7^ w.; and in latitude lO'' s. longitude 5^ w. 

 from London, it has increased from 2° 4- w. to 12° 4 w.; and in latitude 15* n. 

 longitude 20° w. it has increased from 1*^ to 9° w. 



But there is still a more extraordinary ap- 

 pearance in the Indian seas : for instance, 

 under the equator. Where the west varia- 

 tion in the longitude 40° e. is the same in 

 both the above years; and in 1700 the 

 west variation seemed to be regularly de- 

 creasing from longitude 50° e. to the longi- 

 tude 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 va- 

 riation again. 



Such are the irregularities that exj^erience has 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 Dr. Gowen Knight, in the 87th prop, of his treatise on attraction and re- 

 pulsion, that it is influenced by various and dirterent 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. The tables follow. 



