Geological Society of Dublin. 69 



of waves taking place in the solids forming the crust of the earth, 

 and in the fluids incumbent upon it, and resulting from a rupture or 

 disturbance occurring at some one point of that crust. If they sup- 

 posed an earthquake taking place under the sea, three sets of waves 

 started at the same moment from the centre of rupture. One of 

 these was the wave of elastic compression, which was propagated in 

 every direction, and was transmitted with a velocity proportioned to 

 the varying elasticity and density of the crust. The passage of this 

 wave constitutes the true earthquake shock. There was also a wave 

 of sound transmitted through both the solid and the liquid portions 

 of the earth's surface, and also a wave of sound transmitted through 

 the air by the impulse communicated to it from the first-mentioned 

 waves ; and last of all, the great sea wave, produced by the commo- 

 tion at the bottom of the sea, which reached the land long after the 

 shock itself had passed it. Here were three classes of motion, which 

 in general were talked of as connected with a very formidable phse- 

 nomenon, but seldom viewed as movements capable of measurement. 

 It was a fact, that, no matter where an earthquake took place, the 

 wave of elastic compression which was produced actuated every por- 

 tion of the earth's crust, and its occurrence could be detected, by 

 sufficiently accurate instruments, at any part of the globe. 



In some of the observatories connected with the great magnetic 

 survey, now in operation at various points over a great portion of 

 the whole world, and in the organization of which our fellow-mem- 

 ber, Dr. Lloyd, has had so large a share, there existed a necessity 

 for certain magnetic instruments, which were, in fact, though not 

 intentionally so, ready-made seismometers : and in the magnetic 

 observatory of Dublin, Dr. Lloyd has frequently observed a sudden 

 vertical oscillation communicated at the same instant to all the in- 

 struments, and which he believes to be the effect of slight earth 

 tremors, propagated from remote centres of disturbance. 



It would be of the highest service, not only to physics in general, 

 but also to geology, if the circumstances of earthquake shocks could 

 be observed and their velocity measured ; since, if they could tell the 

 velocity of the wave through the earth's crust, and were at the same 

 time to determine by proper experiments the elasticity of the super- 

 ficial rocks of the earth's surface, they should be able to infer, frwm 

 the two data so obtained, the nature of the geological deposits under 

 the bed of the ocean, concerning which nothing whatever was known 

 at present. Topographical geology as yet extends but to a small 

 portion of the land ; future examinations may embrace the whole of 

 this. But of the nature and boundaries of the formations which 

 compose the bed of the ocean, we know absolutely nothing ; so that 

 in fact two-thirds of the whole surface of the globe is in this respect 

 a geological blank. The method of investigation here proposed, 

 however, although it may not give us minute information as to the 

 character of the ocean bed, will enable us to gather some general 

 and certain knowledge of the architecture of those fathomless abysses 

 of the ocean, which no eye can ever behold. He would, therefore, 

 hereafter by the aid of the Council ask, perhaps through the medium 



