1824.] Thirty-Seven Principal Stars. 137 



refraction, and always for aberration, lunar nutation, solar nuta- 

 tion, and variation, ere the index error of the instrument can be 

 found, by a single zero star ; and in proportion as many, or few 

 of these stars are observed, will the index error be well, or ill 

 determined ; and the importance of its being accurately ascer- 

 tained cannot, perhaps, be rendered more evident than by stat- 

 ing, that index error is to observations with the circle, what the 

 clock's error is, to observations with the transit instrument. But 

 lest I should be suspected of exaggerating the difficulties of pro- 

 curing, either the corrections in right ascension, or those of 

 polar distance, I will here present the reader with a specimen of 

 each. 



Greenwich Observations for 1821, Nov. 27. 



a Lyrse. 



Observed right ascension 18 h 30' 46*33" 



Observed polar distance 51° 21' 33*9" 



Barometer 29-54 inches. 



Thermometer 46° 



Correct ion for Right Ascension. 



Equation for aberration, precession, and solar inequality of 

 precession. 



By Maskelyne's Table 17, column « Lyras, Nov. 26 = -f-0-38" 



Dec. 6 = + 0-30 



Therefore diff. for 10 days = -O08" 



And diff. for 1 day = -0-008 



The equation, therefore, for Nov. 27 will be + 0-38" — 0-008" 

 = + 0-372". 



Equation for Nutation. 



By reference to the Nautical Almanac, and by the help of a 

 little calculation, the place of the moon's node on Nov. 27, is 

 10' 29 d 35'. 



By Maskelyne's Table 18, column a Lyra; 10 s 20 d = +0-41" 



11 = +0-30 



Therefore diff. for 10 degrees . . . . = —0-11" 



And diff. for 25 minutes = —0-005 



Now - 0-11" - 0-005" = - 0-105". 



Equation, therefore, for 10 s 9 d 35' = + 0-41" - 0-105" = 

 + 0-305" ; and + 0-372" + 0-305" = + 0-677" = the cor- 

 rection required. 



