JOURNAL 



ASIATIC SOCIETY 



Description of a Persian Astrolabe^ submitted to the Asiatic Society 

 by Mojor Pottinger, By J. Middleton, Esq. Principal of the 

 Governmerd College at Agra. 



The Astrolabe, whose name sufficiently expresses the purpose which 

 it was originally intended to subserve, seems at first to have been of 

 very simple construction, consisting of two concentric rings of brass, 

 the one revolving within the other, upon pivots fixed in their exterior 

 and inner edges respectively. The instrument thus formed, was so 

 placed, that its exterior circle coincided with the plane of the meridian ; 

 the interior one was then made to revolve till the shadows of that part 

 of the limb, towards the subject of observation, overlapped that of the 

 opposite part, when the opening of the rings shewed the meridional 

 distance of the luminary. By a different adjustment of this simple 

 instrument, the zenith distance of the sun, and hence the latitude of 

 the place, could be nearly ascertained. Subsequently, the number of 

 rings of which the instrument was composed, was augmented until 

 not only the meridian, but also the Equator, the Ecliptic, the Colures, 

 &c. were represented, and thus it was when Ptolemy found it, who 

 established the principles, and contrived the means, by which to pro- 

 ject the whole upon a plane surface ; and to this projection he gave the 

 name Planisphere. Of the modification of the Astrolabe the Arabs 

 availed themselves, and were enabled by their improvements in science, 

 especially in Trigonometry, to raise it to a degree of perfection which 

 sufficed, during several centuries, both in Asia and part of Europe, for 

 No. 118. New Series, No. 34. 5 d 



