774 Note on the Compass stars [Sept. 



with their skies their distances are sometimes different which must be 

 known for the purpose of operating. 



Note on the above chapter. By James Prinsep, Sec, As. Soc. fyc. 



The first chapter of the Mohit, as I anticipated, explains all the allu- 

 sions to the stars, the points of the compass, and the methods of mea- 

 suring the latitude, which were so difficult to understand in the chap- 

 ters of voyages first translated ; while the examination of the Arab and 

 Maldive quadrants (if they may be so called) to which I was led in 

 order to understand the nature of the ' celestial inch' or isbd, &c. 

 has prepared me to comprehend at once the descriptions in the present 

 chapter which, as the Baron states, " are quite incomprehensible without 

 the knowledge or sight of the instrument itself, which no doubt must be 

 actually known by Indian or Arabic masters*." 



The first question to be solved is what are the actual stars corre- 

 sponding with the designations adopted in Sidi's work, as well as on the 

 Arabic compass ? The fourth section furnishes the data for the solu- 

 tion of this point, for it contains, not the azimuthal positions of their 

 rising and setting, but their absolute declination north or south of the 

 equator. But to compare these declinations with our present tables al- 

 lowance must be made for the annual variation in declination for the 

 time elapsed since Sidi's tables were framed. To find this epoch we may 

 take the declination of Polaris, i!*^, which is given in the text as N. 

 86° 80', whereas on the 1st January 1839f it is by the nautical alma- 

 nac, N. 88° 27'. The difference, 1° 53' = 6780 seconds, divided by 

 -f-l9".3 the annual variation of this star, gives 353 years prior to 1839 

 as the epoch, or A. D. 1486. Sidi's book was written in 1554, but 

 it was compiled from ten works of preceding authors, five of them 

 ancient, and five modern. The tables he consulted were probably 

 much anterior, perhaps those of Ulugh Beg (A. D. 1437), or of 

 Nasir uddi'n Tu'si', astronomer to the Mongol Halagu Khan at 

 Tabriz in A. D. 1264. It is impossible to expect much accuracy where 

 the text does not pretend to come nearer than the half of a degree, but 

 still as we have sixteen stars we may apply the Bentley method of mi- 

 nimum errors to find the date : 



* On board the Futtle Barry, (Falih-ul bad) I could find none of these in- 

 struments — nor were the points of the ancient compass known — all is now 

 English in Arabic navigation. 



f I make use of this epoch because I happen to have on my table a Green- 

 wich Ephemeris for 183 ( J, and none for the current year. 



