70 Geological Society of Dublin. 



of the General Council of the British Association, that all the observa- 

 tories in connexion with the magnetic survey should, in addition to 

 their magnetic observations, record observations as to the occurrence 

 of earthquake shocks. This might be very readily done, since the 

 magnetic instruments at present in operation in some of the obser- 

 vatories were quite adapted to afford the necessary indications with 

 respect to the shocks, and other instruments required might be easily 

 added, namely, those which it will be necessary to contrive to re- 

 gister the altitude of the crest of the wave of elastic compression, at 

 the moment of its passage — an element in this inquiry of the first 

 importance. 



But it happened, that magnetic observatories which were furnished 

 with these sorts of instruments, did not generally exist in places in 

 which earthquakes were frequent. It thus became necessary to 

 establish, in some places, at least, observatories specially devoted to 

 geological purposes as their primary object. It would be highly de- 

 sirable to have such observatories in the volcanic regions of South 

 America — the land of earthquakes ; since, in addition to observing 

 the velocity of the shock wave, the elasticity of the neighbouring 

 rocks, through which the shock had passed, could experimentally be 

 determined. He conceived that it would be quite possible to get 

 observatories established there, and he should submit to the Council 

 the means by which such a desirable object could most probably be 

 accomplished. Shocks of earthquake were of greatly more frequent 

 occurrence than was generally supposed, and occurred in every part 

 of the world. Violent ones, producing destructive effects, and which 

 were only experienced at limited distances from the centre of dis- 

 turbance, were comparatively rare ; but minor shocks were of almost 

 daily occurrence ; and, indeed, in a district in Scotland, the number 

 of such slight shocks occurring within a certain period had been ob- 

 served. The proposed observatories, therefore, will be by no means 

 unprovided with work. 



There was another class of motion to which the earth's crust was 

 subject, and to which Sir William Hamilton's note related. . Before 

 reading it, he (Mr. Mallet) might perhaps mention that as much as 

 four years ago, upon an occasion of his dining in company with Dr. 

 Robinson, Astronomer Royal of Armagh, he mentioned to him (Mr. 

 Mallet) and others, that the observatory of Armagh had been ob- 

 served to be subject to very slow and minute annual motions, not 

 only to one which was manifested by the whole observatory being 

 lifted in summer and depressed in winter, but also to one by which 

 it appeared to move in azimuth. Dr. Robinson mentioned the fact 

 as one which had not been explained, and it occurred to him (Mr. 

 Mallet), and he stated at the moment, his anticipation, that it would 

 be found that the motions were due to expansions of the earth's 

 crust, caused by the alternations of temperature of summer and 

 winter. Very recently, conversing with Sir William Hamilton on 

 the subject, he (Mr. Mallet) perceived that a similar idea in con- 

 nexion with it had independently presented itself to his mind ; and, 

 in fact, Sir William Hamilton was to be considered as the discoverer 



