336 Mr. Galbraith on the Reduction of the North Polar 



elicited ; but tiiese may be reserved for another communica- 

 tion. The use of the present Table may be shown by the 

 following examples. 



Example 1. Required the mean place of Polaris on the 

 1st of January 1830 from the Greenwich Catalogue, allow- 

 ance being made for the difference between Ivory's and Brad- 

 ley's refractions, the latter being those by which its mean 

 place was deduced. , u 



Polaris N. P.D. by Nautical Almanac 1830 1 35 51-40 

 Correction of this by the table 0'53 



Mean place by Ivory's refractions 1 35 50*87 



Example 2. Required the mean place of Spica Virginis, 

 January 1, 1830. , 



By Nautical Almanac N. P. D 100 16 13*10 



Correction of this by the table + 1*16 



Mean place by Ivory's refractions 100 16 14-26 



Or, Declination 10 16 14-26S. 



Decl. fromBessel (Encke's Ephemeris)... 10 16 17-88S. 



Difference less than Bessel's 3*62 



Difference without the correction 4*78 



Example 3. Required the mean place of Antaros, Ja- 

 nuary 1, 1830. f n 



By Nautical Almanac 1830, N. P.D 116 2 43-30 



Correction from the table 4- 1*60 



Mean place by Ivory's refractions 116 2 44 '90 



Declination 26 2 44-90 S. 



Decl. by Bessel 26 2 49-258. 



Difference after correction 4-35 



without correction 5'95 



Whether corrected or not, these discrepancies appear not a 

 little surprising in the present state of astronomy. 



It will also appear that by the usual mode of determining 

 latitudes by means of the zenith distance, if the object ob- 

 served be to the north of the zenith of Greenwich, the latitude 

 would be too small ; if to the south, the declination being also 

 south, it would be too great by the amount of the error in 

 the place of the celestial object. Hence if extreme accuracy 

 be required in deducing latitudes, &c. the mean north polar 

 distance of the stars in the Greenwich Catalogue should al- 

 ways be corrected by this table, or by some similar means. 



TABLE 



