216 



REPORT — 1861. 



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 tliis purpose, previously to each explosion, the distance of the mouth 

 of the heading was measured with accuracy (which the ground admitted 

 of) from the ilagstaff at W (see Map, Plate II., and Section 1, Plate III.), 

 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 W), 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 formulge, 



i(A + B)=90°-iC, 

 log tan I- (A-B)=log (a-^^) + log tan | (A + B)— log (A + B), 



C being the observed angle, a and b the known sides from flagstaff to O, 

 and from flagstaff to the mouth of the heading. 



Thus the actual range of wave-transit from the focus of each explosion 

 to the seismoscope 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 II.), taken from Mr. Rendel's chart of 1850, 

 published by the Admiralty. Upon it the measured base (A B), and tri- 

 angulation 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. I, 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 junction of the quartz-rock and of the slate, are also marked. The 

 great clay dyke passing through the quartz rock at the quain'ies in rear of 

 the headings is marked by a pair of interrupted lines. 



The Map is to a scale of I| inch to 1000 feet, but is not quite exact as 

 to filling in details on land ; the important distances here concerned are 

 therefore marked in by figures. 



In the opposite Table (p. 217) our chief numerical results are comprised 

 at one view. 



The first result that strikes the eye at once in regarding the Table (p. 217) 

 is, that, with the exception of the experiment No. 1, all show that the transit- 

 rate tends to increase in velocity with the increased quantity of powder 

 fired, — in other words, that the loss of velocity in the same rock is less, in 

 some proportion, as the force of the originating impulse of the wave is greater, 

 and its amplitude greater therefore on starting. 



This is apparent if the uncorrected transit-rates (col. 8) be arranged 

 in the order of increased weights of powder exploded, thus : 



Table II. 



Experiment No. ] forms the only exceptional case, and the departure is 

 not a wide one ; so that the result cannot be viewed as accidental or due to 

 any balancing of errors, but as the expression in so far of a fact of nature. 



