1841.] Description of a Per&imi Astrolabe. 775 



per cccLXXV.M. passuum ortus petatur." Without doubt, these were the 

 Canary Islands, but there is no reason to believe that, by any observa- 

 tions of his, the navigator ascertained them to lie under the first 

 meridian ; it is rather to be suspected, indeed, that from their being a 

 group, and lying in the supposed direction, he assumed them to be 

 such. Pomponius Mela also attempts to identify the Fortunate Isles 

 with the Canaries, but his description, more minute indeed than that 

 of Pliny, is so tainted with incredibilities, as to convince us of tne 

 little reliance that is to be placed on the observations of those who 

 supplied him with information. Some Arabian authors of the twelfth 

 century have got over the difficulty of identification, by asserting, that 

 the " Fortunate Islands" had been, before their time, submerged. How- 

 ever this may be, it may I think be easily shewn, that the first meridian of 

 the ancients could not have passed over any part of the Canary Islands. 



In order to ascertain the first meridian, as implied in their compu- 

 tations, I selected some of the principal places, and thus found it to 

 be about 35° 50' to the west of ours, and thus about nearly 6*^ beyond 

 the most remote of the Canary Isles.* Bagdad for instance, according to 

 Uiug Beg, in whose authority I have much confidence, lies in 80° E. 

 longitude, while its ascertained longitude is with us 44° 30", which gives 

 for the first meridian of the Arabs, a position 35° 30" west of ours, or 

 about 5° to the west of the Canary Islands, and by the whole amount of 

 this difi^erence have the errors of Arabian longitudes been augmented 

 where errors existed, and supposed where they were not. Playfair, for 

 instance, in the introduction to his Geography, while commenting upon 

 their inaccuracies, expresses surprise that they should so far have mis- 

 calculated the longitude of the debouchment of the Indus, which if 

 he had taken their first meridian in place of the assumed one of the 

 Canary Isles, he would have found it pretty exact. 



To our Astrolabe belongs, as already stated, several circular plates 

 of brass, upon which are inscribed stereographic projections of the 



* The following computations confirm this statement : — 





a. Long. 



E. Long. 



Bij^. of Long. 



Average diff. 



Medina, 



... 75° 20" 



39° 20" 



36° 





Damascus, ., 



... 72° 



36° 



36° 



35° 53" 



Ispahan, 



. . . 86° 4U" 



52° 



34° 4U" 





Sheraz, 



. . . 88° 



52° 45" 



35° 15" 





Bussorah, 



... 84° 



46° 30" 



37° Si)" 





