304 REPORT — 1851. 



were formed at Glasthule Quarry in granite, at the same place at which the 

 first preliminary trial blasts were fired, and the same station as then was again 

 chosen for the Seismoseope, being at a range or distance of 150 yards. 



Upon the top edge or lip of the mercurial trough of the seismoseope a 

 thick plate of glass was ground air-tight, and an exhausting syringe was so 

 attached that a pretty good vacuum could be formed between the level plate 

 of glass covering the mercurial trough and the mercury therein, and the 

 whole so arranged that disturbance of the mercurial surface caused the cross 

 \i ires to disappear as usual, when seen through the glass plate reflected from 

 the mercury. 



A wooden box of 3 feet square and 15 inches deep was also prepared, and 

 filled evenly and carefully with well " teased out " curled hair, such as is 

 used for stuffing cushions ; upon the top of which was placed a square flat 

 board of one inch thick, whose surface dimensions were an inch less each 

 way than those of the box inside. The box, when resting level on the rock, 

 was so adjusted that the seismoseope could be sustained in a level position 

 (floating as it were on the curled hair-spring mattress) on the centre of this 

 square board. 



These preparations having been made (the latter end of October), a very 

 calm day was chosen, and the jumper-holes having been charged with about 

 4 lbs. of powder each, the seismoseope was first placed upon the bare solid 

 granite, the plate of glass and the telescopes adjusted, and the air exhausted 

 from above the mercury. Then four of the blasts were fired in succession, 

 and the amount of disturbance of the cross wires noticed. The range was 

 too short to count the time of wave-transit by any method, and the shock 

 was sufficient to cause the cross wires to disappear completely more than once, 

 generally twice, at each explosion. 



The double disappearance is a very interesting fact. It is most probably 

 the transit of the normal, and then of the transversal wave, rendered visible to 

 the eye for the first time, and seems to indicate that by a sufficiently delicate 

 modification of the Seismoseope and chronographic apparatus the interval 

 in time between their consecutive arrivals might be ascertained, and thus 

 much insight gained to the so far obscure or unknown relations that subsist 

 in a vibrating mass between the mutual displacements and the elastic com- 

 pressions of its molecules, in virtue of which, even in homogeneous bodies, 

 the wave becomes one of two sheets. 



This being done, the box of curled hair was placed upon the solid granite, 

 the Seismoseope upon it, the plate of glass removed, and the whole adjusted, 

 the air havnig free access to the mercury. In this condition the other four 

 jumper-holes were fired, and the amount of disturbance of the cross wires 

 observed ; an attempt also was made to note the brief interval of time elapsing 

 between the flash or smoke (observed in the former and latter case by an 

 assistant and chronograph) and the disappearance of the cross wires, when 

 announced by the observer. 



In the former condition of the instrument, the mercury cut off" from any 

 direct contact with the atmosphere and resting on the solid rock, was in the 

 best possible position for being afl'ected by the wave through the earth, and 

 in the worst possible position for being moved by'the wave through the air ; 

 while in the latter condition it was, as far as possible, cut off" from any di- 

 rect contact or hold on the solid ground by the supporting cushion of curled 

 hair, and the mercury itself was freely exposed to the vibrations of the air, 

 while the whole instrument, resting upon a large flat, dry, deal sounding 

 board, was in the best position to be powerfully affected by pulses if commu*. 

 nicated through the atmosphere. 



