500 



sciEJsrcu. 



[Vol. IX., I,'o 224 



At Memphis, Tenn. (590 miles), the signal- ser- 

 vice observer reports a considerable number of 

 stopped clocks, one at 9.54, and the others at 9 55, 

 For some unaccountable reason the seconds were 

 not noted. The phase is unknown. 



The foregoing comprise those time reports which 

 seem to justify the presumption that the errors do 

 not exceed one minute. There are others which 

 are obviously rude approximations, si^'ing exact 

 hours, quarter-hours, or tens of minutes. Tliere 

 are also some which look at first like good obser- 

 vations, but which surely involve some large un- 

 explained error. 



As the discussion of the time data is now pro- 

 gressing, no further comment will be offered here, 

 beyond the remark that there can be no dou bt 

 that the speed of propagation exceeded 3 miles, or 

 5,000 metres, per second. The only questions are, 

 how much this speed was exceeded, and whether 

 the speed along any given line was constant. As 

 regards the latter question, the data are not yet 

 precise enough to justify an opinion. This matter 

 will be inquired into. 



The high rate of propagation will probably 

 prove unexpected to European seismologists. We 

 propose, however, to follow it up with the sugges- 

 tion that it is about the normal speed with which 

 such waves ought to be expected to travel, and 

 that all determinations of the rate of propagation 

 in any former great earthquakes, which are much 

 less than 5,000 metres per second, for normal 

 waves at least, are probably erroneous in propor- 

 tion as they fall short of the Charleston earth- 

 quake. Finding, as the time reports accumulated, 

 that a speed in excess of 5,000 metres was indi- 

 cated, and this presumptio« having become a con- 

 viction, we were led to inquire whether there were 

 not some speed deducible f rom the theory of wave- 

 motion in an elastic solid to which all great earth- 

 quakes ought to approximate. 



In a homogeneous and perfectly elastic solid, 

 the rate of propagation is, according to theory, 

 dependent upon two properties of the medium, — 

 elasticity and density. There are two coefficients 

 of elasticity in solid bodies, one of which meas- 

 ures their resistance to changes of volume, the 

 other to changes of form. Absolute experimental 

 determinations of the values of these coefficients 

 have never been made. If, however, we knew 

 the ratios of these coefficients in one substance to 

 the homologous coefficients in any other sub- 

 stance, and if we also knew the rate of propaga- 

 tion in either of them, the rate in the other would 

 be at once deducible. The rate in steel bars has 

 been the subject of much experimentation, and is 

 given by Wertheiin, whose researches have been 

 as careful as any, at 16,800 feet per second. But, 



as the waves in a steel bar are essentially waves 

 of distortion, he multiplies this result by |/| or | 

 for the normal wave, giving a speed of 21,000 feet 

 per second. The elastic modulus of steel for en- 

 gineering purposes is usually taken to be 29,000,000. 

 The corresponding modulus for such rocks as 

 granite and basalt in a very compact state is 

 about 8.000,000, If we may assume that these 

 moduli are proportional to the two elasticities of 

 the two substances respectively, we can compute 

 the rate of propagation in rock. This assumption 

 may or may not be true ; but we assume it to be 

 so. Let Vs be the rate of propagation in steel, 

 and Vr the rate of propagation in rock, and let eg 

 and er be their true elasticities of volume, and let 

 Ds and Z),- be their respective densities. Our as- 

 sumption is, that 29 : 8 : : eg : Cr, from which we 

 may form the equation, 



Taking the density of steel at 7.84, and of deeply 

 buried rocks in their most compact state at 2.85, 



Vs _ I /^^ 2785 

 1^ — I / 7.84 ^ 8 ^ 



1.15 nearly. 



Taking the rate of compressional waves in steel 

 to be 6,400 metres per second, gives 5,570 metres 

 for similar waves in very compact and dense 

 rock. The corresponding rate for waves of distor- 

 tion would be 4,450 metres. These results are so 

 near to those deduced for the Charleston earth- 

 quake that they seem to be worthy of considera- 

 tion. 



The experimental measurements of the rate of 

 impulses obtained by Milne and Fouque seem to 

 us inapplicable. The elasticity of the surface 

 soil, we think, is no more to be compared with 

 that of the profound rocks which transmit the 

 great waves of an earthquake, than the elasticity 

 of a heap of iron filings is to be compared with 

 that of an indefinitely extended mass of solid 

 steel. The difference is toto coelo. But the rate 

 of propagation is a question of elasticity and 

 density chiefly. The effect of temperature we 

 have not considered. Perhaps the most striking 

 experim?nt ever made with an artificial earth- 

 quake was at the Flood Rock explosion in Hell 

 Gate, near New York, where General Abbott found 

 a speed of propagation approaching very closely 

 to that of the Charleston earthquake. 



The question which is undoubtedly of deepest 

 interest in this connection is whether the Charles- 

 ton earthquake throws any new light upon the 

 origin of such events. While we are not pre- 

 pared to say that absolutely nothing will be added 

 to our information on this question, we are forced 



