128 The Rev. Dr. Wall on the Nature, Age, and Origin of the 



the motion of the same object in a hundred years, and taking the difference. 

 As, then, the error of motion for a hundred years is to the present error of mean 

 longitude, so let one hundred years be to a fourth proportional ; and this will be 

 the number of years that have elapsed since the point of time when the system 

 gave with exactness the mean longitude of the celestial object under examination. 

 Now it is evident that the aim of an Indian astronomer, in constructing new 

 Hindoo tables, must have been to avoid the inaccuracy, ascertained by expe- 

 rience, of older ones, and to make his calculations as to the places of the heavenly 

 bodies agree with actual observation as near as he possibly could. But in adjust- 

 ing the mean motion ascribed to each planet so as to accomplish this end, he was 

 in fact bringing the above-mentioned point of exactness (if I may so call it) near 

 to his own time ; and the more successful he was in his adjustment, the nearer 

 those two points of time must have been to coincidence. We cannot, however, 

 depend upon any single operation for determining the latter time by means of 

 the former. It is far more likely, considering the imperfection of the Hindoo's 

 means, that inevery case there should be a failure of coincidence, the point of 

 exactness for some of the planets, &c. preceding the era of the construction of 

 his tables, and for others following it. The only secure way, therefore, of arriving 

 at the era in question is to calculate several of those points, and the greater the 

 number of the calculations, the nearer must their mean result come to the precise 

 epoch which is the object of our search. 



The method above described has been applied by Mr. Bentley to deter- 

 mining the ages of the two principal Hindoo systems of astronomy, that of 

 Varaha Mihira, which is detailed in the Surya Siddhanta, and is asserted by 

 the Brahmans to have been constructed above two millions of years ago ; and 

 that of Brahma Gupta, which is acknowledged on all hands not to be above 

 thirteen hundred years old. Let us begin with the former system. In the year 

 1799, when Mr. Bentley made his calculations, the error in the mean longitude 

 of the Moon's apogee, as deduced from the tables of the Surya Siddhanta, was 

 4° 15' 28.2 " ; and the error of those tables as to the motion of the same apsis in 

 the course of 100 years is 42' 10.9". As then 42' 10.9" is to 4° 15' 28.2 ", so 

 let 100 be to a fourth proportional, which comes out 605. There had, conse- 

 quently, in the year 1799, about 605 years passed, since the time when the tables 

 of the Surya Siddhanta would have given the mean longitude of the Moon's 



