112 PHILOSOPHICAL TRANSACTIONS. [aNNO I7J4. 



Some Remarks on the Variations of the Magnetical Compass, published in the 

 Memoirs of the Royal Academy of Sciences, with regard to the General Chart 

 *of those Variations made by E. Halley ; as also concerning the true Longitude 

 of the Magellan Straits. By Dr. Halley. N° 341, p. l65. 



The gentlemen of the Royal Academy of Sciences in France, have, for 

 some years past applied themselves, with much candour and diligence, to 

 examine the chart I published in the year 1 701, for showing at one view the 

 variations of the magnetical compass, in all those seas with which the English 

 navigators are acquainted : and, to my no small satisfaction, I find that what I 

 did so long ago, has been since verified by the concurrent reports of the French 

 pilots, who of late have had frequent opportunities of inquiring into the truth 

 of it. So that I am in hopes I have laid a sure foundation for the future dis- 

 covery of an invention, that will be of great use to mankind when perfected ; I 

 mean that of the law or rule by which the variations change, in appearance re 

 gularly, all the world over. Of this I ventured to give my thoughts in N° 148 

 and N° I Qo of these Transactions,* and as yet I see no cause to retract what I 

 there oflTer for a reason of this change ; but of this we might be more certain, 

 had we a good collection of observations made in that ocean which divides Asia 

 and America, and occupies about two fifths of the whole circumference oF the 

 globe. 



In the mean time I cannot omit to take notice of two particulars, seeming to 

 call in question the truth of my map, which I have lately observed in the 

 Memoirs of the Royal Academy of Sciences. The one is in the Memoirs of 

 the year 1700, concerning the variation observed at Paraiba in Brasil, about 25 

 leagues to the north of Pernambouc, by M. Couplet le fils, in words to this 

 effect " On the 20th of May, 1698, having before carefully drawn a meridian 

 line, which I used in my astronomical observations, I observed the declination 

 of the needle to be 5° 35' north west." And the same observer tells us, that 

 he found the latitude of the town of Paraiba 6° 38' 18^. Now it happened, 

 that I was in the river of Paraiba, in the month of March, l6gg, and there 

 fitted and cleaned my ship, so that I had full opportunity to observe the varia- 

 tion both on board and on shore, and found it constantly to be above 4° north 

 east ; so that I am willing to believe this to be an error of the press, putting 

 N. W. for N. E. ; or rather of the memory of M. Couplet, who, it seems, 

 lost all his papers by shipwreck on his return. The like may be said of the 

 latitude of Paraiba, which, though I did not observe myself, yet at the fort of 



• Pa^ 624, voL ii, and 470, vol. iii, of these Abridgments. 



