ON THE TRANSIT-VELOCITY OF EARTHQUAKE WAVES. 215 



right hand, and with the left made contact of the poles of the firing-battery 

 at the same instant that the flag reached the horizontal position. Standing 

 facing me, and as distinctly observable by me upon each occasion as though 

 I had been close beside him, my own eye and attention were directed to 

 Mr. Cousens's left hand ; at the instant that I observed the contact made by 

 him, I released my chronograph, and at once transferred my eye from the 

 eyepiece of the observing-telescope to that of the seismoscope. A moment 

 elapsed before my own eye adjusted itself to the focus of the latter; but the 

 length of transit-period of the wave (always above 4") gave ample time for 

 this, and then at the disappearance of the cross wires, as in the former case, I 

 arrested the chronograph. The only source of time-error introduced by this 

 plan was that of the probability of some slight inequality of speed in dipping 

 the poles to make contact on Mr. Cousens's part (which may be called 

 his personal equation), and the introduction of a somewhat larger value 

 than before to my own personal equation— in the former arrangement that 

 being due to consent between my hand and observation by the eye of 

 one object, in the latter between the hand and observation of tivo objects. 

 As regards the first, several experiments were made by Mr. Cousens and 

 myself at the firing-station, by his repeatedly lowering the red flag and making 

 (the movement of) contact, the contact-maker (fig.2, Pl.IV.) and chronograph 

 being so arraiiged as to register the total interval of time in each case 

 between the first visible motion of the red flag and the completion of 

 contact; others were so made as to register the time between the hori- 

 zontal position of the red flag and the completion of contact. The result 

 gave a minimum error of 0"-009, and a maximum of 0"-017. The mean 

 error, 0"-013, is thus almost equal to the constant due to the contact- 

 maker (in previous arrangement), with this difference, however, that the 

 error in the present case might be either + or — . In twelve experiments 

 nine were -f, or additive ; that is to say, the contact was made more slowly 

 with the left hand than the flag was dropped with the right. The probability 

 is therefore 3 : 1 that the error would be always additive, and would not 

 exceed 0"-013, even if my observation was wholly directed to the flag; but 

 as I directed my attention as completely as possible only to the movement 

 of the contact-making hand, it is still less, and therefore, as not amounting 

 to more than 6 or 7 feet per second in transit-time, may be neglected alto- 

 gether. As regards my own personal equation of observation, it will be seen, 

 on reference to "Second Report," &c. (British Association Report, 1851, 

 p. 305, &c.), of the former experiments at Killiney, where it was ascertained 

 for both observers that its amount is much too minute to enter sensibly 

 into the present results ; and it is needless to say that this is a fortiori the 

 case as respects the time lost in transmission of the galvanic current through 

 the. 12,000 or 13,000 feet of conducting wire. 



The diagrams (Plate III.) give, to one scale, horizontal sections of the 

 several headings from the experiments on which transit-results have been 

 deduced, and a vertical section also of No. 31, quarry No. 9, as illustrative in 

 this respect of all the others. The line of heading, from the face of the cliff up 

 to any focus of charge, turns, it will be seen, thrice at right angles to itself, 

 the object being more effectually to confine the effort of the powder when 

 fired, and prevent the mass of " tamping " from being blown out. Results 

 have been deduced from two headings, each of single focus, two of double 

 focus, one of triple focus, and one of four foci, — the face of the cliff blown 

 out varying, as marked in each case in the figure, from 60 feet to 120 feet 

 in height, and the total weight of powder fired at one time being from 2100 

 lbs. up to the enprmous charge of 12,000 lbs., or nearly 6 tons. 



