Obliquity of the Ecliptic. 243 



This result is confirmed by those of Brinkley, who makes 

 it 23° 28' 17"'54; and of Bessel, who gives 23° 28' 17"'65. 

 The mean of all these three would be 23° 28' 17"'66, almost 

 the same as Bessel's. Since Bradley's determination was 

 23° 28' 18", agreeing exactly with Mayer's, and Lacaille's alone 

 1" more, perhaps the mean of the whole, or 23° 27' 17"*66, 

 might be safely adopted, as the respective differences from this 

 scarcely exceed a tenth of a second ; more especially, if Brad- 

 ley's alone were adopted, to which the corrections above pro- 

 perly apply, it would be 23° 28' 17"'45, still deviating little 

 from the mean which we, on the present occasion, are in- 

 clined to prefer. 



Mean obliquity in 1750 23° 28' 17"*66 



in 1820 23 27 46 '525 



Difference in 70 years 31*135 



Consequently 31"-135-r-70 = — 0"'4448, 



the annual diminution, and agrees with what I have else- 

 where found from the observations of Piazzi, Bradley, and 

 Maskelyne. 



I shall next make a comparison of these results with such 

 observations of the obliquity as appear most deserving of con- 

 fidence. 



Years. 



Authors. 



Obs. Obliquity. 



Computed 

 Obliquity. 



Difference. 



1750 [ 

 .800<J 



1809<j 



1812 

 1813 { 



1820 [ 

 Mean 



Bradley, Mayer, 



and Lacaille ... 

 Maskelyne, Brink- 

 ley, Delambre, 



Cacciatore, Piazzi, 

 Oriani, Arago, 

 Pond, and Bessel 



Oriani, Pond, 

 Brinkley and Arago 

 Pond, from a mean 



]23°28' 17"-66 

 >23 27 55-94 



V23 27 51 -33 



23 27 50-17 

 J23 27 50-50 



J23 27 46-53 



23° 27' 55"-42 



23 27 51 -32 



23 27 50-08 

 23 27 49-64 



23 27 46-52 



— 0"-52 



-o-oi 



-0-09 

 -0-86 



-0-37 









Hence it may be inferred that the mean obliquity for 1750 

 and for 1820 has been well determined, and that the annual di- 

 minution derived therefrom is very near the truth at the pre- 

 sent time. I have reserved the more ancient observations for 

 the following 



2 12 



Tabh 



