596 Transactions. — Chemistry and Physics. 



quite unreliable. In the great majority of New Zealand earth- 

 quakes I find that most of the observations of direction, when 

 properly used, give a very fair rough idea of the position of 

 the origin, especially if that is not too far away. A direction- 

 circle drawn as below, for instance, prevents us from hunting 

 on a false track, and in the present case, after drawing that, 

 I should not expect to find the origin anywhere else but east 

 of Taslnania. 



What is observed is, as Mr. Biggs says, undoubtedly the 

 direction of the movement of the earth- particle, or of what 

 depends thereon — the movement of the ground or of objects 

 resting upon it. But this n:iovement is by no means arbitrary, 

 nor can the movement of the earth be compared to the rolling 

 of a ship on the waves. The earth's crust is an exceedingly 

 rigid and highly elastic mass, not homogeneous certainly, but 

 so nearly so in effect that, as Major Dutton has shown in his 

 celebrated memoir upon the Charleston earthquake, we shall 

 not go far wrong if we treat it as homogeneous. Then, the 

 vibrations from a shock at any point within it must obey the 

 ordinary laws of vibrations. The vibrations set up will in 

 general be of two kinds — longitudinal (that is, to and fro along 

 the line of propagation of the wave, like the vibrations of the 

 air in a tube along which a sound-wave is sent) and transverse 

 (that is, at right angles to the line of propagation, like the 

 small vibrations set up in a stretched string by a blow upon 

 it). For instance, if an earthquake were sent from A to B, 

 the longitudinal vibrations would be backwards and forwards 

 in the directions of the arrow-heads, and the transverse vibra- 

 tions would be to and fro across the line AB, as CO (see 

 Plate LXII.). 



At any distance from the origin, considerable compared 

 with the depth of the actual centrum, the longitudinal vibra- 

 tions will be nearly horizontal ; while the transverse vi- 

 brations will give rase to up-and-down movements, to hori- 

 zontal movements at right angles to those of the longitudinal 

 vibrations, and to movements in directions between those of 

 the up-and-down movements and those of the horizontal 

 transverse vibrations, but always at right angles to the longi- 

 tudinal vibrations, unless there has been reflexion or some 

 other cause of disturbance of the direction of the earthquake- 

 waves. The seismograph record of almost any considerable 

 earthquake will show these several movements more or less 

 distinctly. The disturbing causes are not likelv in the ma- 

 jority of cases to make much difference for an earthquake like 

 the present, of intensity vi. or vii., unless the formation of 

 the underlying strata be very peculiar indeed. 



Generally speaking, the longitudinal waves seem to reach 

 any given place first ; but often only the one kind of vibration 



