444 Translation of the Mohit, [Aug. 



In the description of the islands of the Arabic coast, it will be seen that 

 as the readings of the upper limb increase, those of the lower quadrant 

 decrease, their sum being constantly 14^ issabd. 



Thus, at Saibdn, (Loheia,) the upper reading is 6|, lower 7k, sum 14^. 

 at Jedda, ditto 10 „ 4£ „ 14|. 



It is evident, therefore, that the kids occurs at 7| inch, or 7 issabd 

 1 ssdm, the half of 14^. 



Again, for the zero point we have the following data : — 



true lat. issabd zero point 



By the Jedda latitude, 21° 28'— 10 X 1° 36' (= 16" 00') = 5° 28' 



By the Guardafui, latitude , 12 00 — 4§ X 1 36 (= 6 36) = 5 24 



By Darwesh, or Dorish, 18 30 — 8 XI 36 (=12 48) = 5 42 



By Loheia or Saiban, 15 41 - 6| X 1 36 (=10 48) = 4 53 



ByWasaliat, (Fossailiat,) 17 42 — 7$ X 1 36 (= 12 24) = 5 18 



By Danefc, 19 31 — 8$ X 1 36 (=13 32) = 5 59 



By Haseek sinave, (Harik-simdr), 19 50 — 8| X 1 36 (= 13 56) = 5 54 



The average index error or zero point is thus found to be 5° 31', 

 or in round terms 5^ degrees, to be added to altitudes taken by the issabd 

 scale. The only conjecture we can offer as to the origin of such an arbi- 

 trary arrangement is, that the zero has been fixed at the lowest elevation 

 at which it is safe to trust to the measurement by the polar star : so that 

 mariners, on finding polaris fall below inches, would then know they 

 should take their latitude by Farkadain, or £ and y ursae minoris. 



The zero point of the under quadrant may be easily deduced from the 

 foregoing to be at 28° 18' below the horizontal line. This we may suppose 

 was the highest elevation of the polar star observable by navigators in the 

 Red Sea or in the Persian gulph. Indeed the latitude of Suez and of the 

 mouths of the Euphrates, the most northerly ports visited by their ships, 

 being 30°, the difference between this and 28° 18' is very nearly equal to 

 the north polar distance of polaris (1° 42') : and in the period from January 

 to July it would be only the inferior meridional passage that could be 

 observed. It is generally supposed that the early astronomers regarded 

 the polar star as stationary, and did not trouble themselves to attain even 

 the accuracy we are giving them credit for; but an expi'ession, which occurs 

 in a following page, shows that this was not the case ; since it directs, that 

 1 if it be not time to take the polar star, then another star is to be substi- 

 tuted.' 



As a proof, however, that no great accuracy was attainable, it may be re- 

 marked that the two stars called by the Arabs Farkadain (£ and y ursse 

 minoris) are accounted to have the same altitude; whereas in reality there 

 is a difference of no less than three degrees in their declination ; but it is 

 probable that the altitude was taken constantly by either the upper or the 

 lower star, although we have no data here to decide this point. 



The following table will be found very useful for the conversion of issabds 

 into degrees and minutes. In it the quadrantal difference of the Far- 

 kadain is assumed at 6§ issabd, or 11°+ 5° 30', = NPD 16" 30', which is 

 nearly the average north polar distance of the two stars. 



