326 ULLOA's voyage to south AMERICA. 



Variations obferved by the Author, the Longitude being reckoned from the former Meridian, 



To the above obfervations on the variation of the needle, compared with thofe on 

 the variation chart, firft publiftied by the great Dr. Halley in 1700, and correded in 

 1 744, from other obfervations and journals by Meffrs. Montaine and Dodfon of Lon- 

 don, I fhall add fome reflections, in order to expofe the negligence in conftruding the 

 magnetic needles, i. It appears that the variations obferved by Don George Juan do 

 not agree with mine, which is not to be attributed to a defe6t in the obfervations. This 

 is fufficiently evident from comparing them. The differences between thofe obferved by 

 Don George and thofe on the chart, are nearly every where uniform ; the moft confi- 

 derable being a degree and fifty minutes ; one making the variation 2° 30', and the 

 other a degree only. This probably arofe from the motion of the fhip, which hinders 

 the needle from being entirely at reft ; or from the difk of the fun, by reafon of in- 

 tervening vapours, not being accurately determined, or fome other unavoidable accident ; 

 the error, when the difference is lefs than a degree, being fcarcely perceivable in thefe 

 obfervations. Thus, on a medium, the rational conclufion is, that the needle ufed in 

 thefe obfervations varied a degree and forty minutes lefs than thofe when the map was 

 conftructed. 



The , fame uniformity appears in the differences between my obfervations and the 

 chart ; but it muft be obferved, that having ufed two different needles, the particulars 

 of each nearly correfpond, fo that between the five firft, the greateft difference is of 

 forty minutes, which intervene between the fmalleft difference of 2° 50', and the greateft 

 of 3° 30'. Hence, taking the medium between both, the difference between my ob- 

 fervations and the chart will be 3^ 16', the latter being fo much lefs than the former. 

 The three laft do not want this operation, the difference of 1° 30^ being equal in all, 

 and the variations refulting from thefe obfervations are alfo lefs than thofe delineated on 

 the map ; the variation having paffed to a different fpecies ; namely, from north-weft 

 to north-eaft. This demonftrates, that the firft needle I made ufe of, whether it had 

 been ill touched, or the fteel not accurately placed, varied i^ 30' wefterly lefs than 

 that ufed by Don George Juan ; and as this officer continued his obfervations to the 

 end of the voyage with the fame needle, the difference, which at firft was negative, on 

 the variation changing its denomination became pofitive ; and from my changing inftru- 

 ments, the difference on my fide continued always negative. The reafon of this is, that 

 the difference of the five firft obfervations proceeded lefs from a real difference in the 

 variation, than from the poles of the needle, which was fo far from anfwering exaftly 

 with the meridian- line on the compafs-card, that it inclined towards the north-weft ; the 

 contrary happened in the fecond compafs made ufe of, its inclination being towards the 

 north-eaft ; confequently, whatever the angle of that inclination was, it occafioned a 

 proportionate diminution in the variation of a contrary fpecies. 



Thefe 



