102 Introduction to the Seventh Section 



duck and wood-pigeon ; that the redwings and fieldfares are 

 the most regular and uniform in their appearance and disap- 

 pearance, and most probably never risk the trial of incubation 

 here*; that they quit the country temporarily in severe and 

 long-continued frost through want of food, and return to it 

 again at the approach of more temperate weather ; that the 

 arrival of water-birds forebodes the approach of intense frost, 

 the usual return of the winter-birds, a thaw; that examina- 

 tions of the latter prove them to have taken long flights be- 

 fore their return, and sets the fact of temporary migration 

 beyond the reach of doubt. 



I have then made a digression, and introduced some obser- 

 vations on the singing of birds ; and in a third part, given some 

 additional particulars respecting the different sizes of the ge- 

 nerative organs of birds, as they appear at different seasons 

 of the year. 



XVI. Introduction to the Seventh Section of Bessel's Astrono- 

 mical Observations. 



TTAVING explained in the preceding section the means 

 -*--*- which I have employed to give all desirable certainty to 

 the observations for determining right ascensions, I now com- 

 municate the methods which I have pursued for ensuring an 

 equal degree of accuracy to the declinations. It was neces- 

 sary for this purpose to examine both the instrument itself 

 and the refraction; for, if the hope of advancing further than 

 has hitherto been done, by an accurate knowledge of the in- 

 strument, be well founded, the refraction must likewise be 

 better determined than by preceding observations. It is well 

 known that the refractions hitherto used in this observatory, 

 have been derived by me from Bradley's observations ; but 

 the meridian circle of Reichenbach possesses considerable ad- 

 vantage over the mural quadrant both by the accuracy of 

 single observations, and the possibility of discovering all con- 

 stant errors ; and besides, it admits of any examination which 

 the observer may in future deem necessary; whereas for the 

 Greenwich quadrant the existing number of facts recorded by 

 Bradley in his observations is for ever closed. 



I will endeavour to give a general view of the course of this 

 investigation before I enter into the detail of every part. The 

 errors of the instrument which I have determined so as to be 



* I must be understood by the word " here," to mean that part of Glou- 

 cestershire under niv own observation. 



capable 



