672 ME. MALLET ON THE TEANSIT-VELOCITT OE EAETHQHAKE WAVES. 
equation of observation, it will be seen, on reference to “Second Report*,” &: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 need- 
less 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 XXII.) give, to one scale, horizontal sections of the several head- 
ings, 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 wiU 
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 enormous charge of 
12,000 lbs., or nearly 6 tons. 
It was necessary to ascertain the exact distance in a right line from each of these 
headings, wherever situated, to the observing-station O, at Pen y Brin ; and for this 
purpose, previously to each explosion, the distance of the mouth of the heading was mea- 
sured with accuracy (which the ground admitted of) from the flagstaff at W (see Map, 
and Section I. Plate XXI.). The exact distance of the latter having been previously 
determined from the observing-station O, as already described, the angle of azimuth 
made at the flagstaff by the line of constant range (O AV), and by the line joining the 
flagstaff and mouth of the heading, was observed in each case ; and we thus had the 
requisite data, from which was calculated, by the usual formulae, 
i(A-l-B) = 90°-iC, 
log tan ^(A— B)= log (a— J)+log tan ^(A-f-B)— log {a-\-h), 
C being the observed angle, a and h the known sides from flagstaff to O, and from flag- 
staff to the mouth of the heading. 
Thus the actual range of wave-transit from the focus of each explosion to the seismo- 
scope at O was finally obtained. The positions respectively of each are marked by a 
black dot, and numbered in order of the date of experiment upon the Map (Plate XX.), 
taken from M. Rendell’s chart of 1850, published by the Admiralty. Upon it the 
measured base (A B) and triangulation for obtaining the constant range (O W), and 
for checking that measurement, are marked. The actual wave-paths are therefore in 
right lines, from the dots No. 1, No. 2, No. 3, &c., to the point O. The coast-line and 
position, approximately, of the cliff-faces of the quarries, and the superficial line of junc- 
tion of the quartz rock and of the slate, are also marked. The great clay dyke passing 
* Eeport of Britisli Association, 1851, p. 305, &c. 
