ON THE FACTS OF EARTHQUAKE PH^ENOMENA. 305 



The result of these experiments was, that — 



1. No perceptible difference in time or transit period could be estimated 

 between the flash or explosion and disappearance of the cross wires, in 

 either condition of the instrument. 



2. The actual amount of motion of the mercury was perceptibly greater 

 in the case where the seismoscope stood on the solid rock, with the air 

 exhausted above the mercury. 



3. The cross wires were longer in coming again to rest in the second 

 case, where the instrument stood on the curled hair-box and was freely 

 exposed to air. 



Hence I conclude that in each case the whole effect was due to the stroke 

 conveyed through the earth, and none of it to that conveyed through the 

 air, which seemed too feeble to affect the instrument, though greatly louder 

 than at either Killiney or Dalkey. 



Lastly, to put at rest the possibility of the mercury in the Seismoscope 

 being disturbed by even a pretty loud and near atmospheric concussion, I 

 adjusted it on a surface perfectly at rest in the open air, and fired a rifle gun 

 (No. 14* bore of barrel) with about four drachms of powder for each charge 

 of blank cartridge, repeatedly at various distances and in various directions 

 with respect to the instrument, the report in each case being fully as loud 

 and sharp as that of a common musket. The results were, that up to within 

 SO feet or thereabouts of the instrument no disturbance was produced in the 

 image of the cross wires, when the rifle was held horizontally and loosely in 

 the hands, and pointed away from the instrument. At or within this distance, 

 if pointed towards the instrument and held to the shoulder, or still more if 

 held to the shoulder firmly and pointed vertically upwards, a very slight 

 disturbance could be noficed ; but beyond 30 or 40 feet radius round the 

 seismoscope the effects of the discharge were nil. As in each case these 

 explosions were enormously louder and more ringing than any one of those 

 heard either in the experiments at Killiney Bay or at Dalkey Island, I con- 

 clude it impossible that any movement or commotion of the air due to the 

 experimental explosions can have in any way interfered with or perplexed 

 our results. 



From the construction of our instruments and nature of the experiments 

 already recorded, it will have been remarked, that the degree of accuracy 

 attainable, independently of mere instrumental error, i. e. the probable ob- 

 servational error, depends upon the degree of consent between signal by the 

 eye and hand, as respects both observers, viz. the firing party and the ob- 

 serving party — the first of whom stopped his chronograph by hand on seeing 

 the explosion, while the latter stopped that instrument at the further end of the 

 range, by hand on seeing the disappearance of the cross wires of the Seismo- 

 scope. Some experiments therefore of a sufficiently like character, to deter- 

 mine the degree of accuracy to which this consenting action of eye and hand 

 existed, in both observers, and in what relation to each other, seem necessary. 

 These are as follow. 



Experiments for the Determination of the Personal Equations of the 

 Experimenters. 



These were made with Wheatstone's chronograph and a seconds' beat 

 clock, with graduated arc to the pendulum. 



In the first case one observer counted 30" by the clock, pronouncing the 

 word now at the beginning of the first and end of the last second, while the 

 other obserer started and stopped the chronograph, and then the two ob- 



1851. X 



