1838.] quoted in the Mohit. 779 



It is evident that half an isbd is a great deal too much for the thick- 

 ness of the plates or shortening of the string — I have calculated what 

 it ought to be so as to afford the proper correction for the diminution of 

 the sines, and find it only a twentieth, instead of half, of an inch ; thus, 

 making the isbd = 1° 36' ^ we should obtain the following lengths of 

 the arm or radius ; the isbd being assumed as before at Jths of an 

 inch : — 



No. of 



Angle 



Sine of 



Depth of 



Radi 



us deduced — 



Thickness 



plate or 



subtend- 



ditto. 



the loh in 



D-r 



sin. %/ alt. 



of plate. 



loh. 



ed. 





inches. 





inches. 



inch. 



1 



6 a 26' 



.1120 



3.00 





26.78 



.08 



2 



8 2 



.1397 



3.75 





26.86 



.07 



3 



9 37 



.1670 



4.50 





26.93 



.06 



4 



11 13 



.1945 



5.25 





26.99 



.06 



5 



12 49 



.2218 



6.00 





27.05 



.06 



6 



14 25 



.2489 



6.75 





27.11 



.06 



7 



16 2 



.2761 



7.50 





27.17 



.06 



8 



17 38 



.3029 



8.25 





27.23 



.06 



9 



19 15 



.3296 



9.00 





27.29 



.06 



The next instrument described in the fifth section, does not require 

 much notice since, it is precisely the bilisty, or square rod with a slide, 

 depicted in PI. XL VIII., fig. 2. p. 786, and the mode of laying off the 

 divisions agrees with the plan detailed by my Maldive informant. There 

 seems however to be some unaccountable jumble of the divided rod 

 (gaj) and the knotted string, unless the word translated knot may also 

 signify (as is probable) a division cut on the wooden bar. The applica- 

 tion of the breadth of the tablet for measuring lower altitudes with the 

 same knotted string is of course only an approximation, but quite near 

 enough for practice. The zero point (6 isbdsj is explained to be the 

 lowest altitude of Polaris = 10° 30' -f 3o 30' = 14° ; once more 

 nearly conformable with the latitude of Loheia. 



It is possible that the greater magnitude of the ancient isbd may 

 have proceeded from the practice of taking the polar distance of Polaris 

 as a constant of two isbd : thus in 1394 it would be 3° 52' -r- 2 = 1° 56' : 

 in 1550, 1°33', &c. Even in the chapter before us hardly any two 

 estimates of the isbd agree; in one place 210, in another 224, make 360 

 degrees ; in the division of the gaj and string, the measure will be 

 1° 52' : in other places it is reckoned 1 f degree or 1° 43'. 



The fifth section enlightens us further on the zero point of the isbd 

 scale, which on the former occasion I deduced, from the isbd latitudes 

 of places in the Red Sea*, to be 5° 30' nearly. It says that in taking 

 the altitude of Polaris (always, as I guessed, at the inferior passage) 

 when it comes at last to three isbd (the pole being then five isbd) 

 * Vide vol. V. page 444. 



