690 



years) which is found, if the forreciioii tei'ins are left out of con- 

 sideration, [A?)i], then 



[L)7>] =z £\m — 0.04 — sin <f + 0.21 - sin (f. 



Rn, Jim 



If we accept for the mean distance of the BRADLEY-stars (mean 

 magn. 5.5) according to Newcomb's results : X= -f 0".20, F=r= — 2".60 

 and according to his table on p. 39, as a mean value óto tf = +-0.20, 

 we get Lm = [Am] -f- 0".ll or 



corr. ffp Newc. = + 0".12. 



A separate correction of Newcomb's 7 zones (p. 39) gives tiie result 

 corr. (fp = + 0".ll. 



In the second place we compute the correction which must he 

 applied to the value of A»;, deduced by Dyson and Thackeray from 

 tlie comparison of Groombrtdges catalogue with the second 10 year 

 catalogue. Taking 7.0 as the mean magnitude of the Groombkidge- 

 stars, and accordingly (see Newc. p. 34) adopting for A a small 

 value, putting F= — -ff . 2".60 =r — 2". 00, and accepting {Mantkhj 

 Not. 65, 440) as mean declination of the stars -\- 52°, we find for 

 the coi'rection to be applied to [A??i] : -|- 0".42 sin 52° = -f 0".33. 



In general, if the difference of distances is disregarded, the 

 precessional constant deduced from the right ascensions will be too 

 small if we had used stars of north declinatioii and too large if the 

 stars had south declination. 



b. Determination of the precession from the Declinations. To trace 

 the errors made in this case, by the assumption of equal distances, 

 we must consider the terms containing cos a. We have two prin- 

 cipal terms of this form : A n cos « and — sin ff cos a. Almost al- 



R,. 

 ways, and unless the mean decl. of the stars in question is large, it 

 will be preferable to determine tiie sum of An and the influence 

 of A' and then to substitute the value of A' derived from the R. A. 

 This is also Newcomb's method, and we shall accordingly assume 

 that this has been done and put: 



coetf. of cos a sin d = [A?il 



then, after an easy transformation : 



V Y 

 [Am] =: A« — (0.07 — 0.20 cos^ d) sin d — f 0.04 c<-.s' ds«n d \- 



Rm Rin 



Z 



-\- 0.43 ivs' d sin d . 



Rm 



