LONGITUDE. 



Brought forward 3.10684 Log. of^cof. L = 1279" 

 Brought forward 2.6198S log. of 9 cof. S - = 417 



= 21' 

 = 6 



19" 



57 



+ 



Apparent diftance O 5 



Excefs of this diftance above the reduced 



Diftance reduced © D 



Difference 1422 — = Y 



Or total correftion required. 

 18=42' 3" 

 14 22 



108 27 41 



The only advantage of this method is that it requires only a table of logarithms to five places. 



The true diftance being thus determined, it only remains 

 to find the corretponding time at Greenwich, and to com- 

 pare it with that found on board the fhip. This latter may 

 be found from the altitude of the fun at the moment of ob- 

 fervation : but it may happen that the fun is not favourably 

 fituated at this moment, in which cafe, and indeed generally, 

 the time had better be deduced from folar obfervations made 

 exprefsly for the purpofe ; and which, with a good chrono- 

 meter, may even be made two or three days before or after 

 the obfervations for the longitude, if cloudy weather (liould 

 prevent others being made at a ftiorter interval ; only it muft 

 be remembered, that the deduced longitude will correfpond 

 with that of the place where the error of the chronometer 

 is determined, and not for the place of obfervation. 



The inftruments ufed for thcfe lunar obfervations are 

 fextants and reflefting circles. Under Circle we have 

 already defcribed the reflefting circle of Mr. Troughton's 

 conftruftion, which we conceive to be vaftly fuperior to any 

 fextant for obtaining with accuracy the angle fubtended by 

 the moon and ftar. As each obfervation has three readings, 

 two obfervations (one on each fide) of zero, give fix refults. 

 This inftrument, in the hands of a careful obferver, will not 

 give a greater error than 20", or about ten geographical 

 miles at the equator. The error of the lunar tables may 

 amount to about as much more, and an error in the altitudes and 

 other data, about the fame quantity. Should all thefe con- 

 fpire to produce a total error in the fame direftion, this to- 

 tal error would amount to 60", or 30 miles. We truft, there- 



fore, that the advantage and importance of this method will 

 every day be more and more appreciated ; and that the 

 time will come when no naval officer or mariner of tolerable 

 education will be found ignorant of it. Witli Mendoza's 

 tables, a circls of the above defcription, and a good clu-o- 

 nometer, the longitude may always be determined within 

 thirty miles, and generally within lefs. 



Ssme perfons ftill prefer fextants, from an opinion that 

 they derive tome advantage from length of radius ; but they 

 are fubjeft to errors which have no tendency tocorreft each 

 other, and (ho :ld onl"be ufed for the altitudes, and the cir- 

 cle to be taken in preference for the obfervations of the 

 diftance. When circumftances do not admit of three ob- 

 fervcrs, the altitudes of the fun and moon may be computed, 

 and we are difpofed to think that this would always be the 

 more preferable method, where the obferver is lufEciently 

 ikilful to make the additional computation without fear of 

 miftakes. The altitudes found in this cafe by computing 

 the horary angles are the true akitudes, and muft be cor-._ 

 redfed by applying the refraftion and parallax inverje/y, but 

 for doing this accurately, tables ftiould be computed for 

 reducing Irue dijiances to apparent. However, a little attention 

 to this circumftance will render the whole operation fuffi- 

 ciently eafy, and if the computations are well made, the ac- 

 curacy of this method will probably exceed that of direft 

 obfervation. For the method of making thefe computa- 

 tions, fee Mendoza's Tables, Requifite Tables, &c. Mac- 

 kay's Longitude, &c. &c. 



A Tabz,i; 



