/ i 



® THE,; 



LONDO^EDINBURGH and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[THIRD SERIES.] 



AUGUST 1846. 



XIV. On the Annual Motions of the Earth's Crust. 

 By the Ilev. T. R. Robinson, D.D., fyc. 



To Richard Taylor, Esq. 

 Deau Sir, 



f" SEE in the last Number of the Philosophical Magazine, 

 ■*• that Mr. Mallet has, in a paper on the Secular and Di- 

 urnal Motions of the Earth's Crust, referred to my observa- 

 tions respecting the annual variations of my instruments. In 

 a brief notice, which will be found in the Royal Irish Aca- 

 demy's Transactions, vol. xix. p. 193, I expressed a belief 

 that these are of local origin, being produced by changes of 

 temperature acting in the immediate vicinity of the observa- 

 tory ; and I gave the index corrections of the circle during 

 the year 1839 to show their magnitude and character. The 

 subject of which I was then treating did not permit any fuller 

 explanation ; but the following Table, in which I have com- 

 prised the results of the last eight years, may prove useful to 

 any of your readers who pursue the inquiry suggested by Mr. 

 Mallet. 



Its second column shows for each month the mean eleva- 

 tion of the west end of the transit-instrument's axis, as deter- 

 mined by the level : this operation is in general performed 

 every day on which observations are made; and whenever the 

 adjustment has been altered, the amount of change was care- 

 fully measured. As however in such a case the surfaces which 

 are moved do not assume a permanent bearing for some time, 

 a slight degree of inaccuracy may exist as compared to 



The third column, which gives the change of the horizontal 

 point of the mural circle, or (expressing it similarly to the 

 preceding) the elevation of the northern extremity of a line, 

 on the meridian face of the pier, supposed horizontal at the 



Phil. Mag. S. 3. Vol. 29. No. 1 92. August 1846. G 



