102 Founders of Seesmology.—l. John Michell. 
large quantity of water should be let out upon them suddenly, may 
produce a vapour whose quantity and elastic force may be fully 
sufficient for the purpose.’ Indeed, he remarks, “it is not easy to 
find any other cause capable of ‘producing such sudden and violent 
effects.” 
The fires to which he refers are of the same kind as the fires of 
volcanoes, but, like Humboldt after him, Michell regards the volcanic 
fires as safety-valves, “ for volcanoes, giving passage to the vapours 
that are thus formed, should rather prevent’ earthquakes. If it 
should he asked why we should suppose that subterraneous fires 
exist in the neighbourhood of volcanoes, Michell points to the 
frequent instances of new volcanoes breaking out near old ones, to 
the existence of many volcanoes close together in the same tract 
of country, and to the rarity of isolated volcanoes. 
Michell regards it as very probable that the fires of volcanic 
regions originate in a stratum, like coal or shale, in which pyrites 
is lodged in such quantity that it will inflame of itself, but that the 
quantity varies in different parts of the stratum so that the fires are 
not continuous. The same strata, he remarks, are generally very 
extensive, and they commonly lie more inclining from the 
mountainous countries than the countries themselves. “‘ These 
circumstances make it very probable that those strata of 
combustible materials which break out in volcanoes at the tops of 
the hills are to be found at a considerable depth under ground in the 
level and low countries near them. If this should be the case, and if 
the same strata should be on fire in any places under such-conditions, 
as well as on the tops of hills, all vapours, of whatsoever kind, raised 
from these fires must be pent uy, unless so far as they can open for 
themselves a passage between the strata; whereas the vapours 
raised from volcanoes find a vent, and are discharged in blasts from 
the mouth of them. Now, if when they find such a vent they are 
yet capable of shaking the country to the distance of ten or twenty 
miles round, what may we expect from them when they are 
confined ?”’ Thus, “the most extensive earthquakes should take 
their rise from the level and low countries, but more especially from 
the sea, which is nothing else than waters covering such countries.” 
The manner in which the water contained in fissures reaches the 
subterraneous fires below is explained at some length, but in parts 
with less than Michell’s usual clearness. He supposes the roof over 
a subterraneons fire to fall in. ‘“‘ If this should be the case, the 
earth, stones, etc., of which 1t was composed, would immediately 
sink in the melted matter of tke fire below; hence all the water 
contained in the fissures and cavities of the part falling in would 
come in contact with the fire and be almost instantly raised 
in vapour.” The fall of the roof, Michell admits, could not happen 
accidentally. He connects it with the exnansion of the matter of 
which the subterraneous fire is composed, by which the roof is 
gradually raised and bent until collapse takes place. 
