Geological Society of Dublin. 71 



of this class of motion, since the man who first observed a fact, and 

 at the same time gave a true explanation of it, was entitled to the 

 honour of discovery. Mr. Mallet then read the following note which 

 he had received from Sir William Hamilton, and to which he had 

 referred : — 



"Observatory of Trinity College, March 2, 1846. 



" My dear Sir, — The only thing which seemed to me original in 

 what I observed to you and others at the geological dinner last 

 month, was the proposition for instituting in new, and multiplying 

 in old observatories, observations with a leveling instrument, for 

 the purpose of acquiring accurate data respecting some of the ex- 

 pansions, whether periodical or secular, of the crust of the earth. I 

 thought that by fixing the chief attention on the variations of a long 

 spirit level, very carefully and steadily mounted, and from time to 

 time reversed, as in an astronomical observatory, perhaps with pre- 

 cautions as to original erection and subsequent use, which sidereal 

 checks render not so necessary to the astronomer, and possibly, too, 

 by using two pairs of pillars for two different vertical planes, a gen- 

 tleman might at a moderate expense of money and trouble make in 

 his own lawn or house observations useful to geology ; and if I re- 

 member rightly, under this conception I talked of founding geolo- 

 gical observatories, on which you remarked, that if your paper on 

 earthquakes had been read to the end at the Academy, it would have 

 been found to contain a similar suggestion, though based upon rea- 

 sons not in all respects the same. 



" I also mentioned the fact, that in this observatory, the western 

 end of a transit level, supported on pillars peculiarly favourable for 

 the accurate examination of a point of this kind (see an account of 

 them by Dr. Ussher, in the first volume of the Transactions of the 

 Royal Irish Academy), was always a little higher in summer than 

 in winter ; and that in answer to an inquiry of mine, Mr. Cooper's 

 first assistant had by that morning's post informed me that the axis 

 of the instrument at Markree Observatory showed (such as I conjec- 

 tured that it might) an opposite phenomenon, though this was to be 

 accounted for by mechanical rather than geological consideration. 

 I remember, also, acknowledging that Dr. Robinson had long ago 

 remarked to me that the whole hill on which the Armagh Observa- 

 tory stands is found to have a motion with the seasons, but that I 

 had been in the habit of conceiving Dr. Robinson to deduce this 

 from observations of the azimuth, rather than of the level ; and that 

 my own conjecture, perhaps a very wild one, had been, that Ireland 

 as a whole expanded, and thereby rose somewhat more out of the 

 sea in summer than in winter ; which expansion, if it were admitted 

 to exist, would account for the western end of the astronomical level 

 rising a little on the east and sinking on the west coast of the island. 

 Indeed, as a mode of conjecturally accounting for what has been no- 

 ticed in this observatory, the notion has long been in my mind, and 

 has been put forward by me, though with the necessary diffidence, 

 to some of the astronomical students of the University in one of my 



