300 Mr. R. Mallet on the Seismic Results oltained 



and difFering mucli in depth at various parts. I^ow, as seismic 

 wave-transit is much more rapid through water than through 

 any known rock or other formation, and as the impulse was 

 transmitted from about the same focus through the water 

 and the earth it partially or wholly covered, it is exceedingly 

 improbable that the rapid rate of transit in the water should 

 not more or less influence that through the rock beneath. 

 There then must arise, as it seems to me, great difficulty in 

 knowing what ^^•ave it was that the observers at the different ex- 

 tremities of the difterent ranges noticed by their instruments. 

 It is certain that no two of the observers could have seen the 

 same wave under like modifications, due to the unlike circum- 

 stances of transmission in the different respective ranges. 



ISTor, amongst the numerous sources of uncertainty present, 

 must we omit to notice that there were at least three sets of 

 waves generated at the outset, viz. (1) that of the crackling- 

 explosion itself, (2) that of the fall of the dislocated masses of 

 rock into the cavity excavated beneath the reef, (3) the return 

 fall of the huge mass of water thrown up by the explosion. 

 The further an elastic vibration travels through any part of 

 our earth's crust, the more it gets broken up and diffused as 

 secondary waves ; hence very long ranges are less suited for 

 experiment than those of more moderate length, and moderate 

 charges are on the whole better than very powerful ones. From 

 all these causes, therefore, it seems impossible to decide what 

 was the true origin of the •' tremor" seen in the seismoscope 

 by each respective observer. The tremor at each station is 

 stated to have lasted for several seconds ; but I do not find it 

 stated whether the time of arrival is noted from the com- 

 mencement, middle, or end of this period. 



Nothing is mentioned in the paper referred to as to the 

 nature of the rock or other formations through which the 

 transit-ranges lay. From other sources of information, how- 

 ever, I presume they were all in more or less stratified and 

 disturbed rock, that the line of each ranae was throuo-h differ- 

 ent and partially overlying strata. Now my own experi- 

 ments made in the neighbourhood of Holyhead, North Wales 

 (Phil.Trans. 1861-62), have proved the very considerable differ- 

 ences in transit-velocity in the same rock in the direction of 

 the strata and transverse to the same ; so that from this cause 

 alone it appears to me that the transit-velocity through none 

 of the ranges here referred to could be truly comparable, even 

 if the true transit-rate through any one of them were actually 

 attainable. 



The distances of the various observing-stations from the 

 focus of explosion are stated to have been measured off from 



