ON THE TRANSIT-VELOCITY OF EARTHQUAKE WAVES. 233 



2...V' = 1352 «=--iS==: 1^^=0-791 



/29172bii 1708 



1220 \qQo 



'V/2917262 1708 ^ ' 



The actual velocity of wave-transmission in the slate and quartz together, 

 therefore, was to the theoretic velocity due to the solid material as 



a : ^^2^ or 0-714 : 5-674, or 1-00 : 7-946. 



From which it results, that nearly seven-eigJiths of the full velocity of wave- 

 transmission due to the material is lost bj^ reason of the heterogeneity and 

 discontinuity or shattering of the rocky mass, as it is found piled together 

 in nature. 



This loss would be proportionately larger with still smaller originating 

 impulses, and vice versa, but in what proportion we are not at present in a 

 position to know. 



If we may (or a moment allude to final causes, we cannot but be struck 

 with this beneficent result (amongst others) arising from the shattered and 

 broken-up condition of all the rocky masses forming the habitable surface of 

 our globe, viz. that the otherwise enormous transit-velocity of the wave-form 

 in earthquake shocks is by this simple means so reduced. 



That this retardation is mainly effected by the multiplied subdivisions of 

 the rock, and in a very minor degree by differences in the elastic moduli of 

 rock of diff'erent species, is apparent on examining the Tables IV. and V. of 

 the previous part of this Report referring to the experiments at Holyhead. 



Although, therefore, we are now enabled, from what precedes, to calcu- 

 late values for a, for the slate rocks and for the quartz of Holyhead, sepa- 

 rately, and thus obtain separate values for V, for each of those rocks ; the 

 result would probably be more or less delusive, as we have no possible 

 means of deciding what is the relative amount of shattering and disconti- 

 nuity for equal horizontal distances, in each of these two rocks ; nor what 

 the relative retarding powers, of planes of separation running in variable 

 directions, and at all possible angles, across the line of wave-transit, as 

 compared with their retarding powers, if either all transverse to, or all in the 

 same direction as, the wave-path. 



The greatest possible mean velocity of wave-propagation, in rock as per- 

 fectly solid and unsJiattered as our experimental cubes, is determinable for 

 both slate and quartz in the two directions of transmission, viz. transverse to 

 and in the line of lamination, from Eq. III., and the mean values of L in 

 Nos. 9 and 10, and 11 and 12, Table XII., as follows: — 



ft. per sec. 

 ^ToVniilation""^ '^''^'"^' ^'■•""^^"'■^"} V= 5-674 V 2917262 = 9691 



Mean of slate and quartz m hue ofl ^j rc>-,i /m/.m a r^n? 

 , „. .. ^ ^ V=5-674v 910914 =5415, 



lamination j ^ ' 



both in round numbers; or the transverse is to the parallel transit-rate 

 nearly as 1-8 : 1-0. 



This great difference of velocity, due to the diff"ercnce in the molecular 

 properties of the material of the rocks in their opposite directions, is, as our 

 Holyhead experiments prove, almost wholly obliterated by the vastly in- 



