ME. MALLET ON THE TEANSIT-VELOGITY OF EAETHQHAKE WAVES. 671 
contact-making ; and the new mode was thus continued to the end of the experiments. 
The firing-battery being so disposed upon the sloping brow of the quarry cliff facing 
my station as to be clearly \isible to me, as well as every movement of those employed 
there, a code of signals was arranged between myself and Mr. Cousexs, by which we 
should mutually become cognizant of the state of preparation, &c., and successive acts 
at our respective stations. MTien all was ready at both ends for the explosion, the final 
signal was made by Mr. Cousexs, by elevating a bright-red flag (mounted upon a short 
and light staff) to a vertical position, the lower end resting on a fixed point ; a pre- 
arranged interval of a few seconds (usually 10") intervened, when he dropped the red 
flag, rotating it upon the low'er end of the staff held in the 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 atten- 
tion w’ere directed to Mr. Cousexs’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 eye- 
piece 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. CousEXs’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 me object, in the latter, betw^een , 
the hand and observation of tivo objects. 
As regards the first, several experiments were made by Mr. CousEXS and myself at the 
firing-station, by his repeatedly low’ering the red flag and malting (the movement of) 
contact, the contact-maker (fig. 2, Plate XXIII.) and chronograph being so arranged 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 horizontal 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, O ' OIS, 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 -J- or — . In tw'elve experiments, nine w^ere or additive; that is to say, the 
contact was made more slowly wuth 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 altogether. As regards my own personal 
MDCCCLXI. 4 Y 
