284 



REPORT — 1851. 



it should blow outwards the staves at the bottom, and thus most effec- 

 tually act upon the superincumbent sand. 



The following section shows the dimensions and form (which was a beau- 

 tifully perfect paraboloid) of the holes blown out by each explosion in the 

 sand, after being again partially filled up by the subsequent falling shower of 

 sand. 



Diag. 8, 



f^vs heaP^^ over c 

 ,--' 14 ft. 



Fociu of explosion. 



Each cask was sunk as nearly as possible 6 feet 6 inches below the original 

 surface, and about 1 foot deep of sand was heaped and rammed over the sur- 

 face, making a total depth over centre of explosion of 7 feet 6 inches. 



The next point of arrangement was to devise instrumental means for de- 

 termining with perfect precision, the minute intervals of time that would 

 elapse, — 1, between the moment of ignition of the gunpowder and its com- 

 plete explosion, or the time of " hang fire ;" 2, the interval of time between 

 the moment of ignition and the arrival at the seismoscope of the wave of im- 

 pulse produced by the explosion, the difference in time between these two 

 being the gross time of transit of the impulse through the given distance. 



The beautiful instrument devised by Professor Wheatstone, and called by 

 him the Chronograph, with some suitable additions and modifications, supplied 

 all we required in this respect. 



I had the advantage of Professor Wbeatstone's own suggestions and advice, 

 and a loan of one of his own instruments ; a second I had constructed, by one 

 whose subsequent loss science still deplores, the late Mr. Richard Sharp, 

 chronometer-maker in Dublin. 



As the construction of the chronograph has been fully described by its 

 inventor in the Transactions of the Royal Society, it is needless here to repeat 

 the details. It consists in fact of a small and finely-made clock deprived of 

 its pendulum, but provided with a suitable catch by which the action of the 

 weight upon it can be instantly arrested, or as immediately permitted to again 

 act in giving it motion. The running down of the weight causes the anchor 

 and pallets of the escapement rapidly to pass the teeth of the escapement- 

 wheel, so that the clock " runs down" by a succession of minute descents, 



was heard in the Saxon mines, which are 370 miles away ; here the sound must also have tra- 

 velled through the earth. 



It would be desirable that those who have opportunity would extend our observations of 

 this sort as to the relations between known percussive or explosive efforts, and the transfer 

 of their efforts to ascertained distauces through solid or fluid media, or tlirough air. See 

 also some observations on this subject in former Report on Earthquakes, and compare with 

 the few records we as yet possess of the actual extent of movement of the earth's crust in 

 eartliquake shocks, &c. 



