from the IlaUet^s-Folnt Explosion. 299 



the velocity of seismic-wave transit that the instant of genesis of 

 the wave, or instant at which during the period of genesis the 

 ■wave whose transit-velocity is to be measured starts, should 

 be accurately known ; and for this the impulse generating the 

 wave must result from one single explosion ; and, if possible, the 

 commencement of movement of the rock-mass should be visible 

 from the distant time-observing-point at the extremity of the 

 measured range. Now in this case the impulse was due, not 

 to one explosion, but to thousands of small charges scattered 

 in various directions within a limited but considerable distance 

 from each other, and supposed to be galvanically fired all at 

 the same instant, as no doubt they were, so far as skilful 

 arrangements could permit ; but however apparently simulta- 

 neous as judged by the senses, the impulse generated by such 

 a rapid crackle of small explosions is very different from that 

 produced by a single mass of an explosive agent, or even by 

 three or four such large masses exploded very near each other. 

 I conceive it therefore impossible with sufficient exactitude to 

 assign the instant at which the wave of shock started in this 

 instance ; and to assume the appearance of disturbed water 

 above the seat of explosion, or the shock felt from the ground 

 closely adjacent to it as marking that instant, seems to me, 

 in either choice, to neglect many sources of error. 



The transit-rate was observed at four points, at different 

 distances and situated at various azimuths and under very 

 different surface conditions, along the shores of the bay and of 

 the river Hudson. As to the results of observation made at 

 each of these four stations, viz. (1) Fresh-Pond Junction, 

 (2) Willet's Point, (3) Jamaica, (4) Springfield Junction, 

 situated at the respective distances of 5*134, 8*33, 9*333, 

 12*769 miles from the explosion, it is startling to find that 

 there is not even a near approximation to agreement between 

 them, and that the transit-velocity given by No. 2 (8300 ft.) 

 is more than double that given by No. 1 (3873 ft.), and 

 nearly double that given by No. 3 (4521 ft.), while No. 4 

 (5309 ft.) presents a velocity which, though intermediate 

 between these widely sundered extremes, presents no sem- 

 blance of agreement with any of the three other results. 

 Of the two experiments which present some semblance of 

 approach to agreement, No 1 (3873 ft.) and No. 3 (4521 ft.), 

 I find the first marked as questionable (?) ; and if this be 

 thrown out, there remains not even a semblance of agreement 

 between the other three results. I am therefore compelled to 

 look with entire distrust upon all these results. In every case 

 the w^ave-transit occurred through earth covered for very vari- 

 able portions of the entire range by w^ater variously in motion. 



