786 Note on the Nautical Instruments of the Arabs. [Dec. 



equable increments of the angle of elevation, falling somewhat short of 

 two degrees for each division. Further the highest number, 12, gives 

 nearly the latitude of Calcutta, or 22° 38', the most northerly latitude 

 for which the Maldive navigators have any occasion ; while the lowest 

 mark, 4, gives the latitude (nearly) of the southern point of Ceylon, or 

 the average of the Maldive islands. 



It is a circumstance worth noting, that if the unit had been assumed 

 at 6 diameters instead of 5, there woald have been obtained a series 

 of -divisions almost identical with the issabah of 1° 36' used by the na- 

 vigators of the fifteenth century* according to the Mohit. The series 

 may also be extended both ways without very much deviating from 

 the same progression : thus, commencing with 



Lat. Diff. 

 12 X 6 -i. 16 = 4.50 Cotang. of half angle. 25 u 04' ,_, 

 K " OA 23 32 



15 



4.80 



14 



5.14 



13 



5.54 



12 



6.00 



11 



6.54 



10 



7.20 



9 



8.00 



8 



9.00 



7 



10.29 



6 



12.00 



5 



14.40 



4 



18.00 



3 



24.00 



2 



36.00 



1 



72.00 







infinite 



22 01 



20 28 



18 56 



17 24 



15 48 



14 14 



12 40 



11 06 



9 32 



7 56 



6 22 



4 46 



3 10 



1 36 







1 31 

 1 33 

 1 32 

 1 34 

 1 34 

 1 34 

 1 34 

 1 34 

 1 34 

 1 36 

 1 34 

 1 36 

 1 36 

 1 34 

 1 36 



In this manner a string, or a staff, may be marked off into tangen- 

 tial divisions, equivalent to the issabah, from zero or sixteen issabah, 

 or up to an altitude of 25 degrees, within a limit of error by no 

 means appreciable to the Arab nakhoda, and hardly of consequence 

 to the refined navigator of modern times. Whether the practical rule 

 thus developed was or was not resorted to, it is very plain that it 

 might have been so adapted ; and all the latitudes in Sim's work 

 might have been worked thereby ; and the lower series of divisions 

 might be nothing more than the same divisions numbered inversely 

 on the lower side of the square staff, as will presently be noticed. 



Fig. 2, the bilistyf is an evident improvement upon the original cord; 

 a square rod of ebony being substituted for the stretching cord, and the 

 radius being made to slide thereon at right angles. There is economy of 

 space also, — the four sides of the wooden rod admitting of four series 

 of divisions, adapted to four sliders of different sizes, so as to increase 

 the scale without lengthening the rod inconveniently. Still the string 



See page 445. -j- JuJb 



