Mr. Ivory in reply to the Bulletin des Sciences. 245 



in the Trigonometrical Survey, and will give the difference of 

 longitude on a sphere. 



But even the correct expression of the difference of longi- 

 tude, which we have here investigated, has a disadvantage 

 that makes it improper to be applied in practice ; namely, the 

 result is always affected with the sum of the errors of the two 

 azimuths, which is likewise heightened by the divisor of the for- 

 mula. In the instance of Beachy Head and Dunnose, we have 



\ = 50° 44' 21", m = 96° 55' 58" 



V= 50 37 5 m'= 81 56 53: 



and with these data, the difference of longitude will be found 



equal to, ^ ^ ^ u ^ 



Now I have computed the three following values of the same 

 difference of longitude from independent data; one, by the 

 formula (A) at p. 190 of the last Number of this Journal, which 

 is independent of the azimuths ; and two, by employing the 

 formula (B) at p. 191, combining the azimuth at one station 

 with the latitude of the other, the result not being sensibly 

 affected in this mode of computation by any probable error in 

 the azimuth ; viz. 



1° 27' 5"-62 



1 27 5 -63 



1 27 5 -61 

 It appears therefore that the result obtained by the formula 

 investigated in this article is in defect about ll", which can 

 only arise from an error of about 9" in the sum of the azimuths. 



J. Ivory. 



XLIII. Some Remarks on an Article in the Bulletin des Sci- 

 ences Mathematiques Physiques et Chimiques, for March 

 1828. By J. Ivory, Esq. A.M. F.R.S. 8?c* 



T N the Bulletin des Sciences Mathematiques Physiques et Chi- 

 ■*• miques for March last, there is an article relating to the pa- 

 pers inserted in this Journal, which treat of the attraction of 

 spheroids and the figure of equilibrium of a homogeneous planet 

 in a fluid state. The remarks of the author on the opinions 

 I have ventured to advance on these subjects, seem to call for 

 some notice from me, which I shall study to make as brief as 

 possible. 



With regard to the attraction of spheroids, the usual ground 

 of the dispute is shifted. The controversy has hitherto been 

 confined to the law of attraction that prevails in nature; namely, 



* Communicated by the Author. 



when 



