1841.] Description of a Persian Astrolabe. 763 



terms — horizontal shadow, — vertical shadow, — at their longer and 

 shorter sides, respectively. These rectangles are each divided into 

 two equal parts by the production of the vertical line above. They 

 are next divided on the lower edge from the centre towards the right 

 into twelve, and towards the left into seven equal parts. The vertical 

 edges are also submitted to similar graduation. The meaning and 

 principle of these graduations the next paragraph will explain. 



To the Arab, as to the Hindoo astronomer, the gnomon was an 

 important, if not an indispensable auxiliary. Some divide the 

 shadow into twelve, others into seven equal parts, according to 

 fancy, or the length of the gnomon employed. This scale, whether 

 divided into twelve or seven parts, is of the same length as the gno- 

 mon itself, and is consequently only capable of measuring altitudes 

 within the limits of 0° and 45", or 45° and 90°, according to the 

 situation of the plane upon which the gnomon stands. In order to 

 compensate for this insufficiency, two gnomons are used, one parallel 

 to the horizon, and one vertical to it. At sun rise, it is evident, 

 the shadow of the vertical gnomon is indefinite, and is for long after 

 incapable of being used as a measure of altitude. Again, at the time 

 of sun rise, the shadow of the horizontal gnomon, pointed as it is 

 towards the east, is zero, and gradually increases as the sun ascends, 

 until he has attained the altitude of 45°, when the shadow reaches 

 the limit of its scale, and from that time ceases to be available as 

 a measure of altitude. Whilst the shadow of the horizontal ^/zomo/i has 

 been thus slowly stretching itself, that of the vertical has contracted 

 to the further end of its scale, and is now prepared to perform the 

 functions for which the other ceases to be qualified ; and the same 

 )rocess is repeated, but in an inverse order, till the going down of 

 the sun. 



The scales by which shadow is measured, are sometimes made five 

 [times the length of the gnomon, the shadow being then divided into 

 [sixty and thirty-five equal parts ; but as such a scale could not be laid 

 [down upon the instrument, without causing embarrassment, or injury 

 to its compactness, the following ingenious artifice is resorted to. In 

 [construction of the Astrolabe, the horizontal scales, just described, 

 [are produced to the right and left respectively, the former being then 

 [divided into sixty, the latter into thirty-five equal parts. This done, 



