764 Description of a Persian Astrolabe, [No. 118. 



the edge of a geometrical rule is passed over the centre of the instru- 

 ment and each of the divisions successively, and the points in which 

 the same edge cuts the limb are marked ; the numbers corresponding to 

 them on the original scale are then affixed, and thus the rectilinear scales 

 have been projected into the more convenient form of circular ones, 

 seen in the exterior demi-annulus of the lower limb. It is plain that, 

 by means of these scales and the quadrants of altitude, the height 

 of the sun being given, the length of the shadow may be found, and 

 reciprocally. 



Having thus described, however imperfectly, the astronomical uses 

 of the back of the Astrolabe, I proceed next to its face, wliich exhibits 

 a stereographic projection of the ecliptic on the plane of the equator. 

 1 should here mention, that Plate V. shews the method of construc- 

 tion here employed. If I recollect right, Delembre gives a less ac- 

 curate method by which he believed Arabian astronomers effected this 

 projection, and on this account I consider this a very accurate and neat 

 plate, too important to omit, and by means of which, when fixed over the 

 Planispheres by the common axis, yet left free to revolve, many in- 

 teresting and useful problems, for which globes are used with us, may 

 be readily performed. The numbers on its fancifully formed angular 

 points correspond with those of the list of stars, with which this part of 

 my description will conclude. On the instrument itself, the names, as 

 given in the table, are neatly engraved ; this the flexibility of the 

 Arabic character, and its susceptibility of packing, permitted ; with 

 ours, on the contrary, it could scarce be done, and I have therefore 

 preferred on that, and other grounds, the method of reference by 

 numbers. I believe, observations regarding these stars should more 

 properly come under the astrological head ; but as they are used for 

 ascertaining the time of the night, &c. and as there is besides some- 

 thing very interesting about them, I prefer reversing them from that 

 situation. 



Ulug Beg, whose authority we have given for the position of the stars, 

 was king of Samarcand, and flourished in the early part of the fifteenth 

 century. He was an eminent astronomer ; and the accuracy with which 

 his observations were made, is sufficiently proved by the fact, that on 

 making computation from his data, for the present time, I could readi- 

 ly discover, with one exception, the stars inscribed on the plate. The 



