450 PHILOSOPHICAL TRANSACTIONS. [aNNO I76O. 



3dly, The motion of the earth in earthquakes is partly tremulous, and partly 

 propagated by waves, which succeed one another sometimes at larger and some- 

 times at smaller distances ; and this latter motion is generally propagated much far- 

 ther than the former. 



The former part of this proposition wants no confirmation : for the proof of 

 the latter, viz. the wave-like motion of the earth, we may appeal to many ac- 

 counts of earthquakes : it was very remarkable in the two which happened at 

 Jamaica in 1 687-8 and l6g2. In an account of the former, it is said that a 

 gentleman there saw the ground rise like the sea in a wave, as the earthquake 

 passed along, and that he could distinguish the effects of it to some miles dis- 

 tance, by the motion of the tops of the trees on the hills. Again, in an ac- 

 count of the latter, it is said " the ground heaved and swelled like a rolling 

 swelling sea," insomuch that people could hardly stand on their legs by reason 

 of it. The same has been observed in the earthquakes of New England, where 

 it has been very remarkable. A gentleman giving an account of one that hap- 

 pened there Nov. 18, 1755, says, the earth rose in a wave, which made the 

 tops of the trees vibrate 10 feet, and that he was forced to support himself, to 

 avoid falling, while it was passing. The same also was obser\'ed at Lisbon, in 

 the earthquake of the 1st Nov. 1755, as may be plainly collected from many of 

 the accounts that have been published concerning it, some of which affirm it 

 expressly : and this wave-like motion was propagated to far greater distances than 

 the other tremulous one, being perceived by the motion of waters, and the 

 hanging branches in churches, through all Germany, among the Alps, in Den- 

 mark, Sweden, Norway, and all over the British isles. 



4thly, It is observed in places which are subject to frequent earthquakes, that 

 they generally come to one and the same place from the same point of the com- 

 pass. It may be added also, that the velocity with which they proceed, (as far 

 as one can collect it from the accounts of them) is the same ; but the velocity 

 of the earthquakes of different countries is very different. 



Thus all the shocks that succeeded the first great one at Lisbon in 1755, as 

 well as the first itself, came from the north-west. This is asserted by the per- 

 son who says he was about writing a history of the earthquakes there : all the 

 other accounts also confirm the same thing ; for what some say, that they came 

 from the north, and others, that they came from the west, cannot be considered 

 as any reasonable objection to this, but rather the contrary. The velocity also 

 with which they were all propagated was the same, being at least equal to that 

 of sound ; for they all followed immediately after the noise that preceded them, 

 or rather the noise and the earthquake came together : and this velocity agrees 

 well with the intervals between the time when the first shock was felt at Lisbon, 

 and the time when it was felt at other distant places, from the comparison of 



