Ch. i. south AMERICA. 9 



half, the difference of longitude, by account, would 

 have been only 57''. This error in marking the log- 

 line is common both to the pilots of Spain and other 

 nations; and this, like man\ other faults in naviga- 

 tion, remains uncorrected for want of attention. 



The diftance between the knots on the los-line 

 Ihould contain tIó of a mile, tuppofing the glafs to run 

 exactly half a minute : and though all agree in this re- 

 {peó\, yet not in the true length of the mile, which 

 ought to be determined by the moil exa6l menfura- 

 tions, as thofe of M. Caffini in France, ours in the 

 province of Q,uito, or thofe of M. Maupertuis in 

 Lapland. If the length of the degree be computed ac- 

 cording to M. Caihni's meafurcs, 57060 toifes, a mi- 

 nute or geographical mile will contain 951 toifes, or 

 5706 royal feet, of which tto is nearly equal to 47 

 feet 6| inches; and as the Paris foot is to that of 

 London as 16 to 15 * ; this, when reduced to Engliin. 

 mt'afure, makes near 50 feet 8| inches. 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 exa6i:, when compared to that 

 which has been found from inveñigating the figure of 

 the earth, which is difcovered to be very different from 

 what it has been imagined ; fo that it is not furpriñng 

 that there fhould be found coniiderablc differences in 

 nautical calculations. 



* 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 pre- 

 lident of that i'ociety, the Paris foot is to thai of London as 864 to 

 811, which thews how erroneous thefe are publiflied by father 

 Tofca^. 



* The Paris foot is divided into 12 inches, and each inch into ij lines; 

 wherefore, if we fuppofe each line to be divided in ¿10 parts. 



The Paris foot will be 1440 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 the earth, Part xi. Chap, 5, which 

 ihews the erroneoufnefs of the above. A. 



