776 Description of a Persian Astrolabe. [No. 118. 



sphere, according to the latitudes of certain places. Plate III. is a 

 representation of one of these, and only differs from the others in 

 Polar altitude, or otherwise in the distance of the Pole from the as- 

 signed limit of vision, which in such projections, is supposed to lie 

 considerably below the horizon : here 24°. 



It will be seen from an inscription, at the centre of the plate, that 

 it is characterised not by its latitude alone, but also by a certain 

 measure of time. This was usual among ancient Geometers, who divid- 

 ed, arbitrarily, the earth into a certain number of climates by par..lels 

 to the Equator. The limits of these climates were determined, either 

 by their equinoctial distance, or by the length of their longest day, or, 

 as in the case of our Astrolabe, by both. The column of figures rising 

 upwards from the margin of the plate towards the Pole, belongs to the 

 parallels the latitude, of complements of which are numbered, obliquely, 

 from the Equator northward, on both the east and west sides. The 

 figures indicating the degrees of longitude, are arranged both ways 

 from the meridian to the horizon, and are then continued under the 

 Pole on a semi-parallel of latitude. In addition to the meridians and 

 parallels of latitude, are two concentric circles parallel to the horizon, 

 one above, and the other below it : these are almacanthers of altitude. 

 There are also horary circles, with their corresponding numbers, com- 

 mencing with '' first" at the east point, and continuing round the sphere 

 to the same point, also numbers commencing from at the west and 

 ending with 12 at the east, the reading being backwards. These 

 were called by the Arabs direct or reverse hours, for an obvious reason. 

 The latter arrangement of the numbers arises, as I apprehend, from 

 their being conceived to lie on the side of the sphere opposite to that 

 of the former. 



By means of the Planisphere, the moveable plate, (Plate II.) already 

 described, and the circles of shadow and of altitude, numerous problems 

 regarding latitude and longitude, time, the altitudes of celestial bodies, 

 &c. are readily performed. Indeed, they serve pretty generally, in these 

 respects, the purposes of our globes, with the great drawback, how- 

 ever, that each Planisphere serves only for its own latitude. Hence it 

 is, that each Astrolabe has usually, in addition to their finished spheres, 

 a model plate ; after which others may be constructed. The represen- 

 tation of this is given in Plate IV. It consists of horizontal arcs des- 



