Mr. H. S. Warner on Conjugate Points, 83 



Antrim is so much greater than that of the clay-slate which is 

 found to the south, as not merely to overcome it, but to ex- 

 hibit a large residual action. If we regard the two as result- 

 ants of an inclination of the surface, its intersection with the 



horizon is determined by the equation tang A == ^. _, and 



must make with the meridian an angle of 65° 48' east, which 

 is, I believe, pretty nearly that of the junction of the limestone 

 and slate a little south of Armagh. 



I may add that such changes are not peculiar to this obser- 

 vatory ; in fact few will be found exempt from them. The late 

 Mr. Henderson found that the Edinburgh transit changed so 

 regularly with the temperature, shown by a thermometer sunk 

 twenty-four feet in the rock, that it was a matter of indiffer- 

 ence whether he corrected by the level or the thermometer. 

 Comparing the months of February and August there, W. 

 sinks 5" and N. rises 3" in summer. At Cambridge, the same 

 months give by a mean of 1838 and the four following years, 

 that W. sinks l"*72 and N. sinks 5"*14; and at Greenwich 

 W. sinks about 2" and N. rises about 2"*5. This diversity 

 seems to show that each station has its peculiarities, and that 

 no general system can include the whole. It may also, I 

 think, be inferred, that the agency, whatever it be, is not ne- 

 cessarily of wide diffusion ; and therefore it might be possible 

 in any future construction of a standard observatory, to select, 

 by preliminary experiments, a locality free from such influ- 

 ence. But to do this with effect would require a much larger 

 collection of facts than we at present possess. 



Believe me, 



Observatory, Armagh, Yours sincerely, 



July Jl, 1846. T. R. Robinson. 



XV. On Conjugate Points in relation to Prof. Young's paper 

 on this subject, Philosophical Magazine for August 1845. 

 By H. S. Warner, Esq.* 



I HAVE been induced to consider the subject of conjugate 

 points, from observing that what appears to me to be an 

 important error in Prof. Young's paper " On Imaginary Zeros 

 and the Theory of Conjugate Points" (Phil. Mag., vol. xxvii. 

 pp. 91-97), has not been noticed in any subsequent number 

 of the Philosophical Magazine. 



* Communicated by the Author. [The publication of this paper has 

 been delayed, from a misgiving as to some of the views contained in it, 

 which perhaps mav not meet with acceptance from mathematicians. — Ed.] 



G2 



