670 ME. MALLET ON THE TEAN SIT-VELOCITY OF EAETHQHAKE WAVES. 
made in three instances, noting the time by a delicately made chronoscope by M. Robeet, 
Eue dn Coq, Paris. The results gave 0"'05, 0"'04, and 0"-08 for the time of hang-fire 
respectively, noting from the first visible movement of rock at the face of the heading. 
This would give a mean of 0"‘0566, or very nearly 0"-06 for the time of hang-fu’e, which can 
be viewed, however, only as an approximation. It must vary shghtly with every different 
“ heading,” depending as it does upon a great variety of conditions, but probably much 
more upon the exact proportion subsisting in any given case between the actual resist- 
ance of the rock to the powder employed than upon the absolute quantity of the latter, 
although the total mass of powder burnt is also an element. The greatest observed 
difference between the greatest and least hang-fire amounted to 0"-03, which, converted 
into distance at the mean transit-rate of our experiments, would give possible maxi- 
mum error due to this cause of about 31 feet per second ; the probable error cannot be 
more than about one-half that amount. This correction, converted into distance, is 
also additive. 
By the methods thus described the experiments were commenced and conducted up 
to the middle of 1857. Great trouble and difficulty, however, were experienced ff’om 
tlie outset in keeping the arrangements in working order and so as to be efficient when 
wanted at the very brief notice that could be afforded me beforehand by the officers 
in charge of the works, when suitable headings were about to be fixed. The entne 
line of telegraph wires, the observer’s shed, &c., were exposed to mischief and depreda- 
tion and to injury in that tempestuous place by storms, t&c. The long intervals between 
the experiments involved preparations and adjustment of every part of the galvanic 
apparatus afresh ; upon each occasion, and for the most trifling repairs, workmen had to 
be brought from Conway, or even from Manchester, as also, in every case, to make 
good the branch conductors from the telegraph wires. The length of the range and 
hilly character of the ground also produced much difficulty, in being assured that aU 
was right from end to end against the moment at which the firing was obligatory, as 
well as great personal fatigue at a moment when composed ease and freedom from 
fatigue were most desirable for good observation. 
These difi&culties, in great part foreseen, had early caused me to tru’n my attention to 
the practicability of so adjusting, at the observing-station, a telescope of large field and 
clear definition, and so disposing the Geove’s firing-battery and other apparatus at the 
quarry cliff, that all could be clearly seen from the former point, and the act of making 
contact at the firing-battery obseired by myself with distinctness and certainty, the 
two extremities of the range being thus, as it were, visually brought together. 
Two attempts to experiment in the summer and autumn of 1857, rendered abortive 
by derangements of the galvanic apparatus, caused me finally to abandon it, though 
unwillingly. I found, however, with some satisfaction, that, subject to the possible 
fatahty of a cloud settling over the quarry cliff, and so shutting it out from sight just at 
the critical moment, the telescopic arrangement, on trial, really seemed to offer quite as 
accurate results as the more complex method, and more difficult to manage, of galvanic 
