52 2%e Causes and Phenomena of Earthquakes. 



of Natural History," during 1833, 1834, and 1835, " On certain 

 (then) recent meteoric phenomena, vicissitudes in the seasons, 

 and prevalent disorders, contemporaneous, and in supposed con- 

 nection, with volcanic emanations," in which, of course, earth- 

 quakes had their share. 



Subsequent discoveries have, no doubt, modified some of my 

 views and deductions ; but it was from a familiarity with such, 

 topics, that I ventured to accede to the request made to me at 

 our July meeting, that I would discuss the phenomena of the 

 earthquake which shook the country on the 18th June last. 



This I have now connected with all the other recorded shocks 

 in Australasia, felt since the foundation of the colony to the 

 present time. 



In treating on so much more extended a subject, I consider it 

 necessary to enter at considerable length into certain explanations 

 of the theoretical origin of earthquakes, and to mention the facts 

 that have been observed in relation to their character. 



It is, however, first of all, necessary to give a definition of an 

 earthquake, which, for conciseness, I select from Mr. Mallet's 

 Essay in the Admiralty Manual of Scientific Enquiry (p. 207). 

 He defines it to be the transit of a wave of elastic compression 

 m any direction, from vertically upwards to horizontality in any 

 azimuth, through the surface and crust of the earth, from any 

 centre of impulse, or from more than one, and which may be 

 attended by tidal and sound waves, dependent upon the impulse 

 and upon the circumstances of position as to sea and land. 



As some persons assume that it is to earthquakes alone the 

 elevation of portions of the surface of the globe is due, it will 

 require a certain amount of investigation of the principles laid down 

 by seismologists, before we can show how far such an opinion is 

 or is not supported by evidence. 



As, moreover, an elevation of our own coasts has been con- 

 jectured to be still going on, a brief investigation of the kind may 

 not be out of place on the present occasion, although it may be 

 well to premise, what will again be alluded to, that the effect of 

 earthquakes is generally rather to depress than to elevate. 

 Fortunately, the subject of elevation in connection with earth- 

 quakes has been successfully treated by a physicist of high order, 

 my late friend, Mr. Hopkins, of Cambridge ; whilst the general 

 facts of the phenomena of earthquakes have received great 

 illustration from the labours of Mr. Mallet, of Dublin. To both 

 of these sources I shall apply in the course of my remarks, as 

 well as to other authorities deserving notice, in preference to any 

 crude or undigested arguments resting solely on my own views or 

 calculations. 



It would be impossible to discuss tke earthquake of the 18th 

 June, and those Avhich have preceded it since the Colony was 



