348 Volcanoes and Earthquakes 



third, and so on until they come to a vent, which is either 

 open or weak enough to yield to the pressure. In this way an 

 earthquake and an eruption may be in intimate connection 

 with one another, although a considerable interval of time 

 intervene between the occurrences, and the localities affected 

 be at great distances from each other. And it is possible that 

 some connection of this kind may have existed between the 

 earthquake of Antioch and the eruption of Vesuvius, both 

 having been extreme in their violence. Indeed, the whole 

 series of disturbances, commencing with the earthquake in 

 California and terminating with the eruption of Vesuvius, 

 all of which occurred in April 1872, may possibly find an ex- 

 planation under this theory. 



The effect of sudden relief of pressure in weakening the 

 walls of vessels explains many cases of explosion which other- 

 wise appear anomalous. Thus, high-pressure boilers have been 

 frequently observed to burst at the moment when the engineer 

 turns on the steam. 



Postscript. The subject of this paper was suggested by the 

 extraordinary sequence of seismic and volcanic phenomena which 

 took place in the month of April, 1872. It included disastrous 

 earthquakes in California, on the Gold Coast of Africa, and at 

 Antioch in Asia Minor, as well as the great eruption of Vesuvius, 

 rendered more celebrated by the courage and tenacity of Pro- 

 fessor Palmieri who remained in the observatory and continued 

 his work, though completely isolated and exposed to the risk 

 of being overwhelmed at any moment. 



I was at that time preparing for my work on the "Chal- 

 lenger" expedition which sailed at the end of the year 1872. 

 During it volcanic and perhaps seismic phenomena were likely 

 to present themselves; but I little expected my claim for the 

 seismic effect of sudden relief of pressure to be made good, and 

 still less to be able to demonstrate it by direct experiment. 



Yet this is what happened on 27th March, 1873. In the 

 experiment of that date the pressure suddenly relieved amounted 

 to over five hundred atmospheres and the body on which its 

 seismic effect was exhibited was a stout copper tube. 



The incident is described in the first page of the next paper. 



