1903.] on Recent Volcanic Eruptions. 233 



" fire on the sea," occurring in the black cloud which overwhelmed 

 the windward side of that island. We examined them carefully, and 

 are quite clear that they were electric discharges. The scintillations 

 in the body of the cloud became less numerous and more defined, and 

 gradually took the form of vivid flashes of forked lightning darting 

 from one part of the cloud to another. When the cloud had got 

 within perhaps half a mile or a mile of us — for it is difficult to esti- 

 mate distances at sea and in a bad light — we could see small material 

 falling out of it in sheets and festoons into the sea, while the onward 

 motion seemed to be chiefly confined to the upper part, which then 

 came over our heads and spread out in advance and around us, but 

 left a layer of clear air in our immediate neighbourhood. It was 

 ablaze all the time with electric discharges. 



As soon as it got overhead, stones began to fall on deck, some as 

 big as a walnut, and we were relieved to find that they had parted 

 with their heat and were quite cold. Then came small ashes and 

 some little rain. The cloud was also noticed at Fort de France. It 

 was described as like those in the previous eruptions, but was the 

 only one in which electric scintillations had been noticed. Two 

 unbiassed observers, who had seen it and that of May, declared this 

 was the larger of the two. 



As to the mechanism of the hot blast and the source of the power 

 which propelled it, both Dr. Flett and I are convinced of the inade- 

 quacy of previous explanations, such as electricity, vortices, or ex- 

 plosions in passages pointing laterally and downwards, or explosions 

 confined and directed down by the weight of the air above. Such 

 passages into the mountain, which, to be effective, would require to 

 be closed above, do not exist in the case of the Soufriere, and we are 

 not aware that they have been observed in Mont Pelee, and as to the 

 weight of the air, this did not prevent the explosions in the pipe of 

 the Soufriere from projecting sand and ashes right through the whole 

 thickness of the trade-winds until they were caught by the anti-trade 

 current above and carried to Barbados. Moreover, the black cloud, 

 as we saw it emerge from Mont Pelee, seemed to balance itself at the 

 top of the mountain, start slowly to descend and gather speed in its 

 course, and the second incandescent discharge followed the same rule. 

 We believe that the motive power for the descent was gravity, as in 

 the case of any ordinary avalanche. 



The accepted mechanism of a volcanic eruption is that a molten 

 magma rises in the volcano chimney. It consists of fusible silicates 

 and other more or less refractory minerals, sometimes already partly 

 crystallised, and the whole highly charged with water and gases, 

 which are kept in a liquid state by the immense pressure to which 

 they are subjected. When the mass rises near the surface and the 

 pressure is diminished, the water and gases expand into vapour and 

 blow a certain portion of the heavier and less fusible materials 

 to powder, or, short of this, form pumice stone, which is really 

 solidified froth, and they are violently discharged from the crater. 



R 2 



