M. Bessel on the Declination of the Stars, 433 



servations might also be thereby obtained. I know, iiuleed, 

 that the refraction cannot be considered rigidly correct, either 

 by the exterior or by the interior thermometer; but as the 

 refraction on the outside of the observatory regulates itself 

 according to the exterior thermometer, and can only be altei'ed 

 by the difference of temperature within the observatory, this 

 alteration (sometimes + sometimes — ) may be in other re- 

 spects equally as great for stars on the zenith as for stars in 

 any given altitude, and consequently upon the whole can be 

 but very trifling; therefore there remains no other method, 

 but to be guided by the data of the exterior thermometer. If 

 the surface of the warmed air in the observatory were calm 

 and horizontal, it would be otherwise; but this is not the 

 case. 



In the letter, from which you have given an extract in the 

 28th Number of your Astronomische Nachrichten, I observe 

 that Mr, Pond's catalogue approximates nearer to mme, if it 

 be reduced with the refraction, which I have derived from my 

 observations. But I think that no one should allow himself 

 to make this alteration, unless he can prove from the Green- 

 wich original observations, that the stars below the pole re- 

 duced by the same refraction, give corresponding results. 

 This, the author of the account in the Philosophical Magazine 

 doubts, when he says, that the continued use which Mr. Pond 

 makes of Bradley's tables of refraction, proves that Mr. Pond's 

 observations correspond with this table. I cannot decide 

 about this ; but I cannot help obsei*ving, that I have obtained 

 a greater refraction from Bradley's observations than Bradley 

 himself: and that this latter appears not to be reconcileable 

 with the observations of that great astronomer, when examined 

 with great accuracy. Were it, on the contrary, reconcileable 

 with the latest Greenwich observations, then it would appear 

 to me as a striking coincidence of heterogeneous circum- 

 stances. 



Mr. Pond has had the goodness to send me, through the 

 hands of Dr. Tiarks, a still later copy of his catalogue ; but it 

 differs very little from that compared in the 28th Number of 

 the Astronomische Nachrichten, and therefore does not alter 

 materially the result produced from that comparison. 



Vol.Gl. No. 302. June 1823. 3 I LXXXIX. True 



