VOL. LXXXI.] PHILOSOPHICAL TKANSACTIONS. 3g 



cision, to enable us to found a general rule on. Three of these routes lead 

 across the Great Desert, or that between Aleppo and Bussorah ; the other 2 are 

 across the Little Desert, or that between Aleppo and Bagdad. 



The first of the Great Desert routes was traced by a Mr. Cannichael in 

 1751 ; and his journal manifests a great degree of ingenuity and perseverance in 

 this way. The author declares, that he was determined to keep a register of the 

 courses by a compass, and to compute, comparatively, if not absolutely, the in- 

 termediate distance on each course ; by counting the steps or paces of the 

 camel on which he rode, during a certain interval of time ; and afterwards mea- 

 suring a number of them on the ground. And though Mr. Carmichael failed 

 in the attempt to ascertain his road distance by this method, yet his process has 

 furnished others with the means of ascertaining the whole distance in the aggre- 

 gate, and of proportioning the parts throughout. For, as the direct distance is 

 given by the celestial observations, and a complete traverse table by the journal, 

 the data are perfect. And when the reader is informed that Mr. Carmichael's 

 whole line of bearing, by compass, between Aleppo and Bussorah, nearly 720 

 British miles, coincided with the bearing line given by the celestial observations ; 

 by which it appears that the error could amount only to the mean quantity of 

 the variation throughout, which might have been from 6 to 7° at that time 

 (1751) ; he will give Mr. Carmichael credit for much general accuracy. And it 

 is not improbable, that even a considerable portion of the above error may have 

 arisen from the imperfection of his instrument.* 



The 2d journal was kept by Colonel Capper, in 1778} and was published 

 several years ago ; and the 3d, which contains little more than the time in detail, 

 was communicated by Mr. Hunter, who crossed the desert in 1767. The time 

 given between Aleppo and Bussorah, by these journals respectively, is as follows : 

 by Mr. Carmichael 322 hours ; Col. Capper 310; Mr. Hunter 2994-. But this 

 difference arose chiefly from the variations in the route across the Chaldean 

 Desert, between Mesjid Ali and Bussorah. Mesjid Ali, or All's Mosque, is 

 situated at about 4- of the distance, and as nearly as possible in the line of di- 

 rection between Aleppo and Bussorah ; and is a sort of land-mark to the caravans 

 which pass the common boundary of the Arabian and Chaldean desarts. Now 

 that portion of the Desart route between Mesjid Ali aiid Bussorah, being sub- 

 ject to great variation in the track, as appears by the journals of different 

 travellers, while the much larger portion of it, between Mesjid Ali and Aleppo, 

 is very nearly the same at all times ; it is very clear that this latter portion fur- 



* I find, by Mr. Drummond's chart of the road between Aleppo and Antioch (174-7,) that the 

 variation was then about 6° westerly. This is proved by comparing his magnetic bearing line be- 

 tween those places, with that given by tlie difference of latitude. In the head of the Gulf of 

 Persia, the variation was 7° in 1785. — Orig. 



