1«G 



February 9 th. 



SIR Wm. R. HAMILTON, LL.D., President, in the 

 Chair. 



Richard Cane, Robert Franks, Charles W. Levinge, 

 James Corry Sherrard,PierceMorton,and Stephen O' Meagher, 

 Esqrs., were elected Members of the Academy. 



Mr. Robert Mallet read part of his paper on the Me- 

 chanics of Earthquakes. 



The author first notices the various instances recorded of 

 an apparently vorticose motion having occurred in earth- 

 quakes, as evidenced by the twisting displacement of objects, 

 such as the superimposed stones of obelisks, &c., and then 

 proceeds to demonstrate that the conclusion adopted by Mr. 

 Lyell and other authors, that such a vorticose motion actually 

 takes place, does not follow from the premises, and is incon- 

 ceivable and impossible in many respects. 



He proves that the twisting displacement is due to a mere 

 alternate, straight-line motion, given by the earthquake- 

 shock to the base upon which the displaced body rests. The 

 insistent body is moved by the adhesion of its base, and its 

 inertia, acting through its centre of gravity, will cause the 

 body to twist whenever the point in the base, at which all 

 the adhesion may be supposed to act, and which the author 

 calls " the centre of adherence," lies either to one side or 

 the other of a vertical plane, passing through the centre of 

 gravity of the body twisted, and being on the line of motion 

 of the base. 



The alternate, straight-line motion having such great ve- 

 locity, yet within narrow limits, as thus to move heavy bodies 

 by their inertia, and which constitutes the earthquake-shock, 

 the author defines as the passage of a wave of elastic com- 



