VOL. LI.] PHILOSOPHICAL TRANSACTIONS. 46\ 



are to the place whence they have their source; but at great distances from it, 

 they may rise so little, and so slowly, as not to be perceived, but by the motions 

 of waters, hanging branches in churches, &c. The vibratory motion occasioned 

 by the first impulse of the vapour, will be propagated through the solid parts of 

 the earth, and therefore it will much sooner become too weak to be perceived 

 than the wave-like motion ; for this latter, being occasioned by the vapour insi- 

 nuating itself between the strata, may be propagated to very great distances, and 

 even after it has ceased to be perceived by the senses, it may still discover itself 

 by the appearances before-mentioned. 



Sect. 4. — All earthquakes derived from the same subterraneous fire must 

 come to the same place in the same direction ; and those only which are derived 

 from different fires will come from different points of the compass; but pro- 

 bably as it seldom happens that earthquakes, caused by diff-erent fires, affect the 

 same place, we therefore find in general that they come from the same quarter; 

 it is not however to be supposed that this should always be the case, for it will 

 probably sometimes happen to be otherwise; and this is to be expected in such 

 places as are situated in the neighbourhood of several subterraneous fires; or 

 where, being subject to the shocks of some local earthquake of small extent, 

 they now and then are affected by an earthquake, produced by some more dis- 

 tant, but much more considerable cause. Of this last case we seem to have 

 had some instances in the earthquake of Nov. 1, 1755, and those local ones 

 which succeeded it. 



As we may reasonably infer from many earthquakes coming to the same place, 

 from the same point of the compass, that they are all derived from the same 

 cause, and that a permanent one; so we may reasonably infer the same thing 

 also from their being propagated with the same velocity ; but this argument will 

 still come with the greater force, if it be considered that the velocity of any 

 vapour, which insinuates itself between the strata of the earth, depends on its 

 depth below the surface: for the deeper it lies the greater will be its velocity. 

 We may therefore conclude from the sameness of the velocity of the earthquakes 

 of the same place that their cause lies at the same depth ; and from the inequa- 

 lity of the velocity of the earthquakes of different places, that their causes lie at 

 different depths. Both these are consistent with the supposition, that earth- 

 quakes owe their origin to subterraneous fires, since the strata in which these 

 subsist may be easily conceived to lie at different depths in different parts of 

 the world. 



Sect. 5. — From the same cause we may easily account for those local earth- 

 quakes, which succeed the greater and more extensive ones. If there are many 

 subterraneous fires subsisting in different parts of the world, the vapour coming 

 from one fire may well be supposed, as it passes, to disturb the roof over some 



