34ft ULLOA's voyage to south AMERICA. 



makes it ; which is however generally followed in this voyage. According to the 

 obfervations of Father Laval, rnade at Martinico, the difference of longitude is ^^° B' 4^" y 

 according to thofe of Father Feuillee, ^^^ 19'. This error in a great meafure 

 proceeds from a want of accuracy in the log-line ; for had the pilot of the Conquiftador, 

 who found the fame defe£l: in his calculations, made the diltance between the knots 

 of the log-line thirty Englifh feet, inftead of forty-feven and a half, the difference of 

 longitude, by account, would have been only fifty-feven degrees. This error in mark- 

 ing the log-line is common both to the pilots of Spain and other nations ; and this, like 

 many other feults in navigation, remains uncorrected for want of attention. 



The diftance between the knots on the log-line fhould contain -j-4.^ of a mile, fup- 

 pofmg the glafs to run exaftly half a minute : and though all agree in this refped:, yet 

 not in the true length of the mile, which ought to be determined by the moft exaQ: 

 menfurations ; as thofe of M. Caffini in France, ours in the province of Quito, or 

 thofe of M. Maupertuis in Lapland, if the length of the degree be computed accord- 

 ing to M. Caffini's meafures, 57,060 toifes, a minute or geographical mile will contain 

 951 toifes, or 5,706 royal feet, of which ^4^ is nearly equal to forty-feven feet fix 

 inches and a half; and as the Paris foot is to that of London as 16 to 15*; 

 this, when reduced to Engliih meafure, makes nearly fifty feet eight inches and a 

 quarter. And this is the true diftance between each knot on the log-line. 



This menfuration, which fhould have been hitherto the rule obferved, is not exad:, 

 when compared to that which has been found from inveftigating the figure of the 

 earth, which is difcovered to be very different from what it has been imagined ; fo that 

 it is not furprifing that there fhould be found confiderable differences in the nautical 

 calculations. ^ 



The Author's Journal^ on board the Incendio, 



Having fet fail on the fame day, namely, the 28th of May 1735, and fteered fouth, 

 between fifty-two and fifty-fix degrees wefterly, we perceived on June 2d, about fix 

 in the evening, the ifland of Savages, one of the Canaries ; and on the 3d we faw Te- 

 neriffe. I found the difference of longitude between Cadiz and Naga-Point to be 11° 6\ 

 which agrees with the Englifh and Dutch charts, but differs a little from the true 

 longitude determined by Father Feuillee at Loratava, in the fame ifland of Teneriffe. 



On the 4th, we had fight of the iflands of Palma, Gomera, and Fer ; but again 

 loft fight of them on the fifth. On the 29th about noon, we made Martinico, and 

 continuing our courfe, paffed between that ifland and Dominica. The difference 

 of longitude between Martinico and Cadiz bay, accerding to my reckoning, was 

 S7^ 5', one degree more than San Telmo's chart makes it. But it is proper to ob- 

 serve, that in order to eftimate my courfe, and avoid the danger of finding a great 

 difference at making land, I followed two different calculations, one according to the 

 meafures commonly given by pilots to the diftance between the knots on the log-line, 

 of forty-feven Englifh feet and a half, and the other by reducing them to forty-feven 



* According to the late regulation of the Royal Society of London, and the meafures fent by it 

 to the Academy of Sciences at Paris, and with which I was favoured by Martin Folkes, Efq. the worthy 

 prefident of that fcciety, the Paris foot is to that of London as 864 to 811, which fhews how erroneous 

 thefe are publifhed by Father Tofca f . 



"J- The Paris foot is divided into twelve inches, and each inch into iweh-e lines ; wherefore, if we fuppofe each line to be divided 

 Itt 310 parts, the Paris foot will be 1 440 parts, the London, 1350. 



Thefe proportions were fettled by the Royal Academy of Sciences at Paris, in their treatife of the figure and magnitude of thecartli. 

 Part jd. Chap. .5, which ftiews the erroneoufnefs of the above. A. 



royal 



