ON THE FACTS OF EARTHQUAKE PH^ENOMENA. 281 



strument resting on solid granite, the temperature of the mercury and instru- 

 ment being 64P Fahr. The following gives some of the results, being one 

 set of observations : — 



No. of Experiments. Cross wires observed by 



1 230 beats in 60". 



2 230 „ 



3 231- 



4 231- 



5 229+ „ 



6 230 „ 



7 230 



8 228+ „ bad observation. 



9 229 + 



10 231- „ 



From this it may be concluded that 230 beats per minute just keep the 

 cross wires out of view. The time of wave production and transit then is 

 half this, or 



fin'' fin" 

 -2r — = "^ =0-13 of one second ; 

 230 X 2 460 



but as the image of the cross wires is reflected from the centre of the trough 

 or from a point half way of its length or breadth, one half of the foregoing 

 is tlie constant of correction due to wave-transit of the seisraoscope, or 



^i^^=0-065" in time ; 

 2 



and as this will, in every observation with the seismoscope, appear to delat/ 

 the arrival of the earth-wave at the instrument, this time converted into di- 

 stance must be added to the rate of earth-wave transit otherwise obtained. 



We now return to the preliminary trials as to the charge of powder re- 

 quired for each mine in the sand. 



Twelve of the two-pound tin plate cartridges, before described, and one of 

 Mr. Bergin's large batteries of six cells being prepared, (of which, however, 

 only two cells were found requisite to fire the powder,) on the 25th of 

 October 1849, the seismoscope was adjusted at the terminal north stake, 

 and a cartridge was buried 4 feet in the sand, at the distance of half a mile, 

 keeping it about 60 yards clear of the half-mile stake (to right or left), and 

 fired. The two pounds of powder exploded, but was unable to lift out any 

 sand at that depth, or to produce a shock sensible at the distance of half a 

 mile. A second cartridge was buried 3 feet and fired ; it blew out a column 

 of sand, but no pulse was perceptible at the seismoscope, nor was any sound 

 heard there through the air. A third cartridge was then buried 3 feet at a 

 quarter of a mile distance from the seismoscope, but no pulse was perceptible 

 on several repetitions. At one furlong therefore from the instrument another 

 was buried 3 feet and fired ; this was found on repetition to be distinctly 

 perceptible by the seismoscope, causing the cross wires to disappear in the 

 most distinct manner. 



A rough approximation was thus made as to the quantity of powder re- 

 quired ; for as the intensity of the pulse will vary, with the same original im- 

 pulse, nearly as the square of the distance inversely, so it may be inferred, 

 that to produce equal pulses at variable distances, the original impulse must 

 vary in the same ratio ; and taking the impulse of fired powder to be directly 

 as its weight, if 2 lbs. produced a wave visible with the seismoscope, at ^th of 

 a mile it would require (8)^ x 2 to produce one equally visible at a mile, or 

 128 lbs. 



