470 PHILOSOPHICAL TRANSACTIONS. [anNO IjOO. 



one, by which these earthquakes can be known from those which owe their 

 origin to shallower fires, it must be very difficult to distinguish them with cer- 

 tainty, as it is almost impossible to distinguish the difference of the time of 

 their happening in different places, when the whole perhaps is comprehended 

 within the space of 2 or 3 minutes ; possibly however some of the earthquakes 

 which we have had in England may have been of this class. 



Sect. 7. — If we would inquire into the place of the origin of any particular 

 earthquake we have the following grounds to go upon : 1st, The different di- 

 rections in which it arrives at several distant places : if lines be drawn in these 

 directions, the place of their common intersection must be nearly the place 

 sought: but this is liable to great difficulties ; for there must necessarily be 

 great uncertainty in observations which cannot, at best, be made with any 

 great precision, and which are generally made by minds too little at ease to be 

 nice observers of what passes ; also the directions themselves may be somewhat 

 varied, by the inequalities in the weight of the superincumbent matter, under 

 which the vapour passes, as well as by other causes. !2ndly, We may form 

 some judgment concerning the place of the origin of a particular earthquake 

 from the time of its arrival at different places ; but this also is liable to great 

 difficulties. In both these 'methods however we may come to a much greater 

 degree of exactness by taking a medium among a variety of accounts, as they 

 are related by different observers. But, 3dly, we may come to the greatest de- 

 gree of exactness* in those cases where earthquakes have their source from under 

 the ocean ; for in these instances the proportional distance of different places 

 from that source may be very nearly ascertained by the interval between the 

 earthquake and the succeeding wave : and this is the more to be depended on, 

 as people are much less likely to be mistaken in determining the time between 

 two events which follow one another at a small interval than, in observing the 

 precise time of the happening of some single event. 



Let us now, by way of example, endeavour to inquire into the situation of 

 the cause that gave rise to the earthquake of Nov. 1st, 17o5, the place of 

 which seems to have been under the ocean, somewhere between the latitudes of 

 Lisbon and Oporto, (though probably somewhat nearer to the former,) and at the 

 distance perhaps of 10 or 15 leagues from the coast. For, Jst, the direction 

 in which the earthquake arrived at Lisbon was from the north-west ; at Ma- 

 deira it came from the north-east; and in England it came from the south-west; 

 all of which perfectly agree with the place assumed. 2dly, The times in 

 which the earthquake arrived at different places agree also with the same 

 point. And, Sdly, the interval between these, and the time of the arrival of 

 the subsequent wave, concur in confirming it. That all this might the better 

 appear, Mr. M. subjoined the following table, assuming the point. 



