720 



Observations on the Herat Astrolabe, described in No. 1 18, of the Jovrnal. 

 By the Rev. J. S. Pratt, Chaplain to the Right Rev. the Lord Bishop 

 of Calcutta. 



I feel ashamed that I should have kept the brass Astrolabe you lent 

 me so long, without having more to say upon it than I have. But, 

 I am sorry to tell you, that incessant occupation has prevented my 

 giving any time to its examination since the first day that I brought it 

 home. On that day I measured some of the arcs, and tried to discover 

 the use of the curves drawn upon the smooth face, but could not ascertain 

 their use. In fact, it would require a long and close examination 

 and measurement of these instruments, and an intimate acquaintance 

 with the Hindoo and Arabic methods of calculation and observation, 

 which I have no time to acquire, I regret to say — to lead one 

 to indulge any hope of discovering the use and method of handling 

 these Astrolabes. The difficulties that stand in the way of discover- 

 ing the method of using these instruments, and the object for which 

 they were constructed, seem to be the following : — 



1. An ignorance, an entire ignorance of the object in view ; though it 

 seems most probable, that the object was to calculate the dates of festivals. 



2. The want of a complete knowledge of the principles upon which 

 the instruments were constructed. 



3. An ignorance of the extent to which the numerical constants used 

 in the application of their principles were erroneous. 



4. The imperfection of workmanship in the instrument itself. 

 Perhaps I may presume to say, that the author of the communication 



to the Asiatic Society's Journal, No. CXVIII, entitled "A description of 

 a Persian Astrolabe," meaning, I believe, the one I now return to 

 you, does not attempt to give an explanation of the use of the curve 

 lines, which intersect the circular arcs concentric with the instrument 

 itself. Indeed, these lines are wrongly drawn in the diagram. It seems 

 most probable, that they serve some important office in the calcula- 

 tions and observations for which the instrument was designed; and 



