VOL. LI.] PHILOSOPHICAL TRANSACTIONS. 405 



SO light a vapour, there will now be no want of fluidity in the matter it rests 

 on, and the difference of specific gravity between the two, instead of being small, 

 will be very great; hence, if any part of the roof gives way, it must immedi- 

 ately fall in, the vapour readily rising and taking its place; and a beginning being 

 once made, a communication will be opened with numberless clefts and fissures, 

 that must occasion the falling in of vast quantities of matter, which, as soon as 

 the vapour can pass round them, will want their support: then will follow the 

 great effects already described. Now while the roof is raising, the waters of the 

 ocean ov^er it must retreat, and flow from thence every way; this however being 

 brought about slowly, they will have time to retreat so gently, as to occasion no 

 great disturbance; but as soon as some part of the roof falls in, the cold water 

 contained in its Assures, mixing with the steam, will immediately produce a va- 

 cuum, in the same manner as the water injected into the cylinder of a steam 

 engine, and the earth subsiding, tmd leaving a hollow place above, the waters 

 will flow every way towards it, and cause a retreat of the sea on all the shores 

 round about ; then presently the waters being again converted by the contact of 

 the fire into vapour, together with all the additional quantity which has now an 

 open communication with it, the earth will be raised, and the waters over it will 

 be made to flow every way, and produce a great wave immediately succeeding 

 the previous retreat. 



Sect. 3. — That great quantity of water, which we have supposed to be let out 

 on subterraneous fires, and by that means to produce earthquakes, will supply 

 us with a reason why they observe a sort of periodical return. This water must 

 extinguish a great portion of the burning matter, in consequence of which it 

 will be contracted within much narrower bounds; and though the eflects before 

 described could not take place at first, but by the great extension of the heated 

 matter, yet after they have once taken place, they may well continue to do so 

 for some time; for the great disturbance in the first instance, by the foiling in 

 of a great part of the roof, must render the frequent communication between 

 the fire and water not only very easy, but almost unavoidable; and this will con- 

 tinue to be so till the roof is well settled, and the surface of the melted matter 

 sufliciently cooled, after which it may require a long time for the fire to heat it 

 again so much as will be necessary to make it produce the former eflects. Now, 

 as the matter has been more or less cooled, or as the combustible materials are 

 with more or less difliculty set on fire again, as well as on account of other cir- 

 cumstances, the returns of these effects will be later or earlier; but though they 

 will not, for this reason, observe any exact period, yet they will generally fall 

 within some sort of limits, till either the matter that occasions them is con- 

 sumed, or till the fires open themselves a passage, and become volcanos. 



Sect. 4. — It has been already intimated, that the most extensive earthquakes 



VOL. XI. 3 O 



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