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Full text of "Conjectures concerning the cause, and observations upon the phaenomena of earthquakes"

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Concerning the 




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UPON THE 












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ARTHOUAKE 





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Particularly of 



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That great Earthquake of the firfl of November i/j^, 
which proved fo fatal to the City of Lifbon, and 
whofe EfFeds v^^ere felt as far as Africa, and more 
or lefs throughout almofl all Europe. 



By the Reverend JOHN MICH ELL, M. A 

Fellow of Queen s-College, Cambridge, 



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Read at feveral Meetings of the Royal Society; 





LONDON: 

Printed in the Year M, D C C. L X.' 



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Conjectures concernin 




the Cause 



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AND 



Observations upon the Ph^enomena 



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O F 




A R 




H Q^U A K 





INT 




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C T I O N. 



Read Feb. 28. 7 ArT. I . 



March 6. 13. 
20. 27. 1760. 




has been the ge 



neral 



opi 



niofi of philofophers, that 

earthquakes owe their origin to fome 



fudden explofion in the internal parts of the earth. 



This 



opinion IS very 



agreeable to 



the , phenomena. 



which leem plainly to point out fomething of that 
kind. The conjectures, however, concerning the 
caufe of fuch an explofion, have not been yet, I think, 



fufficiently fupported 




fads; nor have the more 



particular effeds, which will arife from it, been 
traced out; and the connexion of them with the 
phasnomena explained. To do this, is the intent of 
the following pages ; and this we are now the better 

enabled to do, as the late dreadful earthquake of the 



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ift of NoveinBer 1755 fupplies us with more * fa(^'Si; 

'and thofe better related, than any other earthquake 
of which we have an account. 



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That thefe concuffions fliould owe their origin to 




2. 
fomething in thi|;,air,as it has fometimes been imagined,, 

feems. very iirfe correfpond with the phsenomena. 
Thifj- 1 apprehend, will fufficiently appear^ as thofe 
Dlifenomena are hereafter recounted ; nor does there 



apfear to be any fiich certain and regular connexion, 
between earthquakes and the ftate of the air, when 



they happen, as is fuppofed by thofe vvho hold this. 

opinion. It is faid, for inllance, that earthquakes 
always happen in calm ftill weadier : but that this is- 
not always fo, may be feen in an account of the. 

-f- eiarthquakes in Sicily of 165)3, "where we are told, 
the "fouth winds have blown very much, which flill 
have been impetuous in the mod fenfible earth- 



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quakes,^ and the like has happened at other times. 
3. Other examples to the fame purj)bfe we have 

In an account of the earthquakes that happened in 
New England in 1727 and 1728 -, the author 




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Many 



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Tran 



*' Earthquakes, 



The 
The 



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are defirous of being acquainted with this ftibjed). The author^ 
of it has given us, befides the aforefaid fafis, z very judicious- 
abridgment of ten of the moft confiderable v^riters upon the fub- 
*je£t. I have ta6:en the greateft part ofrrtyauthorities either from 
tliis author, or*the Philofophicai Tranfadions, that thofe ^ Who 
would 'wifli to examine them, may have an opportunity of doing 
itthe more eafily J fome things only, which were not to be met 
with in thefe, and whidh "yet were necefiary to my purpofe, I 
have^been obliged to feek for elfewhere. 



Tianf. N'^aoT. orvol.ii. p^4:08. Lo!wth 




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which, fays, tliat lie could neither obferve any cbn- 

between the weather and the earthquakes 



nexion 



nor any prognoflic of them y for that they happened 
alike in all kinds of weather, at all times of the tides, 

and at all times of the moon *. 

4. If, however, it ihould flill be fuppofed, not- 
withftanding thefe inftances to the contrary, that 
there is fome general connexion between earthquakes- 



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vet, 



ihe weather, ait the time when they happen, 

it is far more probable, that the air 




ihould be afFedled by the caufes of earthquakes, 'than 
that the earth ihould be affe(3:ed in fo extraordinary 
a manner, and to fo great a depth; and that tliis. 



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Tranf, N^ 409. or 



vol. 



vi, part ii 



Earnests Abridgment. — To 

opinion of Monf. 

occafion, in the following manner. 
*« Seneca/ tell us, that earthquakes a: 



we 



who exprelles 



may 

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add 



202. 



upon 



the 
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Ariftotle, Pliny, and 



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" ferene air. This is, indeed, often the cafe, but not always 



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examination 



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it. Some 



adarkfky, lightenings, and fudden ftorms, as the forerunners 



^^ of earthquakes." 



Then relating fome inftances of fhocks that 



happened in calm and ferene weather, he adds, 'f -On 



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hand, it appears, from the examples, which we have before 
related, that many earthquakes have happened at the time of 



great rams 



*fi atmofphere. 



y certain prognoflic of them in the ftate o 

See Memoires Hijiorlques et Fhyftques fi 

Mvnf. Bertrand^ a la Hay 



This 



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author, in thefe fefifible memoirs, has obliged the public with a 
circumftantiaraccountof all thefaftshe could colled, relating ta 
the earthquakes of Switzerland, or thofe of other places, th^t 
feemed to be conneSed with them. The whole feems to be done 
with care and '^fidelity, and without the leall attachment to any 



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rs^nd all the other clrcumftances attending tliefe mo 
tlons, ihould be owing to fome caufe refidin 
the 




air. 



f. Let us then, rejecting this hypothefis. 




that earthquakes have their origin under ground, 
we need not go far in fearch of a caufe, whofe real 
exiftence in nature we have certain evidence of, and 
which is capable of producing all the appearances of 
thefe extraordinary motions. The caufe I mean is 
fubterraneous fires. Thefe fires, if a large quantity 



of water fhould be let out upon them fuddenly, may 

produce a vapour, whofe quantity and elaftic force 
may be fully fufficient for that purpofe. The prin« 
cipal fadts, from which I would prove, that thefe 

iires are the real caufe of earthquakes, are as follow. 

^ 

Section!. 

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6. Firft, The fame places are fubje<a to re- 
turns of earthquakes, not only at fmall inter- 
vals for fome time after any confiderable one 
has happened, but alfo at greater intervals of 
fome ages. 

7. Both thefe fadts fufficiently appear, from the 
accounts we have of earthquakes. The tremblings 

and fhocks of the earth at * Jamaica in 1692, at 
* Sicily in 169^5, and at * Liibon in lyff, were re- 
peated fometimes at larger, and fometimes at fmaller 
intervals, for feveral months. The fame thing has 
been obferved in all other very violent earthquakes. 
At f -Lima, from the aSth Odober 1746, to the 




* See the accounts of thefe in the Philof. Tranf. 

f See Ant9nio d'Uiba's Voyage to Peru, paU ji. bopk i. cb. 7 



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yji^y (the time when the account oF 

them was fent from thence), there had been num- 
bered no lefs than 45"! Ihoeks, many of them Httle in- 
ferior to the firfl great one, which deilroyed that city. 
8. The returns of earthquakes alfo, in the fame 

are confirmed by 



P 



ces 



diflances of time 



hiilory 




Minor, have fuffered 



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and many parts of Aiia 
them, in many different 



ily has been fubjed: to them, as far back 



the remains even of fabulous hiflory 



of 



Lifbon did 



firil: time in 



inform us 
feel the effects of them for the 

75S' Jamaica has frequently been 



oubled with them,, fmce the Engflilli firfl fettled 



ther 



e 



and the Spaniards, wh 




wer 



there befor 



ufed to build their houfes of wood, and only 
flory high, for fear of them 
the parts adjacent. 



Lima, Callao, and 
were almofl totally deflroyed by 



them twice, within the compafs of about fixty years 
fcarce any building being left flandingi and the latter 



b 




both times overflowed by the fea 



were 



thefe the only inffcances of tlie like kind, which have 
happened there-, for, from the year 15-82 to 1746, 
they have had no lefs than fixteen very violent earth- 

befides an infinity of lefs confiderable ones 3 

firfl fettling there, were 

the old inhabitants, when they faw them 




and the Spaniards 



told 




building high houfes, that they were building their 



own fepulchres -f- 



5>. Secondly 



* See the place above- quoted. 



What, IS here faid, is taken from d'UlIoa's Voyage to Peru, 

liflory and Philofophy of Earth(juakes, the Philof. Tranf. £2'<:. 

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p, Secondfy, Thofe places that are In the neigh- 
bourhood of burning mountains, are always 
fubjedt to frequent eairthquakes j and the erup- 
tions of thofe mountains, when violentj are ge- 
nerally attended with them. 

10. Alia Minor and Conftantinople may be looked 

upon as in the neighbourhood of Santerini. The 
countries alfo about * ^.tna, Vefuvius, mount Haecla, 

&c. afford us fufficient proofs to the fame purpofe. 



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But, of all the places in the known world, I fuppofe 
no countries are fo fubjed: to earthquakes, as -f Peru, 

Chili, and all the weftern parts of South America; 
nor is there any country in the known world fp full 



of volcanos: 



for, throughout all that long range 



of mountains, known by the name of the Andes, 
from 45 degrees fouth latitude, to feyeral degrees 



north of the line, as alfo throughout all Mexico, 
being about 5000 miles in extent, there is a con- 
tinued chain of them J. 




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The motion of the earth in earth 
quakes is partly tremulous, and partly propa 

waves, which fucceed one 
at larger and fometimes at fmaller 






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With. 



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Memoires ft 



are mentioned above 130 repetitions of earthquakes, that have 
happened, within the compafs of 960 years, in Switzerland* 
* See many inftances of this in vol. ii. of Lowthorp's Abr. of 



Tranf. 



+ Monf. 



that fcarce a week pafles without earth- 



quakes in fome part of Peru, See Hift. of Earthq. p. 205. 
% See the Maps of thefe countries, Condamine's 

the Maranonj Acofta's Nat» Hift, of the Ifldiesj ^e 




diftances 



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propagated much farther than the former. 

The former part of this propofition want 

for the proof of the latter, viz. 



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confirm ation : 

^wave-like motion of the 

:many accounts of earthquakes 

able in the two, which happened at Jamaica in 



the 



th, we may appeal to 

was very remark- 



687-8 and* i6g 



in an account of the former 



faid, that a gentleman there faw the ground 




the fea in a 



wave, as the earthquake pafTed 
aiong," and that he could diflinguifh the effects of it, 

to fome miles diflance, hy the motion of the tops of 



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the trees on the hills 
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faid, « the ground heaved and fwelled 



like a rolling fweUing fea 



fomuch, that people 



could hardly Hand upon their legs by reafon of 



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The fame has been obferved in the earth- 
quakes of t New England, where it has been very 
Temarkable. A gentleman giving an account of one, 

that happened there the 1 8th November 1755, fays, 

the -earth rofe in a wave, which made the tops of the 

trees vibrate ten feet, and that he was forced to fup- 

port himfelf, to avoid falling, whilfl it was palTing.^ 

1 4. The fame alfo was obferved at J L^on, in 

the earthquake of the ifl November 1755, as may 

be 



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p. 410 



Phil. Tranf. N« 209. or vol. li. Lowthorp's Abridgmfint, 



Tranf. vol. I. p. i, ^c 



t See the accounts colleaed together, in the 49th 



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p. 315 



faid, «' A moft dreadful earthquake ihook 



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to 

be plainly colteded from . many of ' the accounts tEatt 

have been publifhed concerning it, fome of which^ 
alHrm it exprefly: and this wave-like motion was> 
propagated to far greater diftances than the other 
tremulous one, being perceived by the motion of wa^ 
ters, and the hanging branches in churches, . through ; 
all Germany^ amongft the Alps, in Denmark, Swer 
den, Norway, and all over the British ifles. 

15. Fourthly, It is obferved in places, which are^ 
fubjed to frequent earthquakes, that they ge- 
nerally come to one and the fame place, from^ 

the fame point of the compafs, I- may adda 
alfo, that the velocity, with which they pro 

ceed, (as far as one can colle(5t it from the ac- 
counts of them) is the fame 5 but the velocity 
of the earthquakes of different xountries is very 

different. 

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16. Thus all. the fiiocks, that fiicceeded the firfl 

great one atLiibon in 1755, as well as the firft itfelf, 

came from the * north- weft. This is alTerted by 
the perfon,,who fays, he was. about writing a hiflory 
of the earthquakes there.: all the other accounts alio 
confirm the fame thing y. for what fome fay;, that they 
came from the north, and others, that they came 



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** fhort, but quick vibrations. 



foundations 



« tion changed. 



' perceptible paule, the nature of the mo* 

every building v^as tofled like a waggon 

rough Hones, vvhich l^id in ruins aim oft 



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Gibraltar, fee Hift, and Philof, of Ba 



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22. 



See Fhilof, Tranf. val. xlix, p 



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from the weft, cannot be looked on as any reasonable 

to this, but rather the contrary. The ve- 



objedion t© this, 
locityairo, with which they were.all propagated, was 
the fame, being at leaft equal to that of found; for 
they all followed * immediately after the noife that 
preceded them, or rather the noife and the earthquake 



came together : and this velocity 




y well 



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the intervals between the time when the firft fhock 
was felt at Liibon, and the time when it was felt at 
<3ther diftant places, frord the comparifon of which. 



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have travelled at the 



of more 



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•f* twenty miles /?^r minute. 

17. An hiflorical account of the earthquakes, which •' 
have happened in J New England, fays, that, of 

five conliderable ones, three are known to have come 
from the fame point of the compafs, n)iz. the nortli- 

weft: it is uncertain from what point the other two 

came, but it is fuppofed that they .came fi-om the fame 

with the former. The || velocity of thefe has been 

much lefs than that of the Liibon earthquakes : this 
appears from the interval between the preceding noife, 

and the ihock, as well as from the wave-like motion 
before- mentioned. 



.* 



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See Philof. Tranf. vol. xlix- p. xr*- or Hift. and Philof. of 



Earthq. p. 315. 
f See Art. 97, 



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See Philof. Tranf. 



pcopa 
by no 
fhock 



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means be owing to any caufe rending 



tbcy af-e 

they can 
for any 



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locity neither greater nor lefs dian that of founds ; that is, at the 



of about thirteen jniles * 



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8. All tHe greater earthquakes, that Have Been' 



felt at * JamaiGaj feem 




the 



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have come from the fea, and,, paffing by 



Port-Royal, to have gone northwards. The velocity, 
of thefe alfo v^^as far fhort of the velocity of the Lif 



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bon earthquakes. 

o. The earthquake of -j^ London, on the 8th ofl 



March i7fO,. was 




to move 




eaft to 



weft. 

thing happened 



Ii have been credibly informed, that the fame: 

a. flight fhock, which was felfc 

there in the laft century, as the perfon, who told me 
this, had an opportunity of obferving; for being, by 
accident, in a fcalemaker's fliop at the time when it 
happened,, he found that all the fcales vibrated from. 

eaft to weft. 

20. All the fhocks that have been lately felt at 

Brigue in Valais, have hkewife come from the fame 

point of the compafs, viz. the fouth J. 

Fifthly y The great Liibon earthquake has been 



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fucceeded by feveral local ones fince 
of which has been much lefs. 



the 



a 2. Such were 



thofe on the borders of France and 
in Barbary, ^c. 



the earthquakes in Switzerland 




3 



thofe 



209 



I^owthorp's Abr. p. 4i©> ^^" , -n. -i r t- r 

t See Hift. and Philof. of Earthq. p. 250. or Philof. Tranf. 



i-L_p 



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The 



t See Philof. Tranf. vol. xlix. p. 620. 
obferved at Smyrna alfo, fee PhUof. Tranf. N° 495. o^ Martyns 

Abr. vol. X. p. 526. ■ , . r.' -1 r T- r 

[I See the accounts of thefe colleaed together in Pmlof. 1 rani. 

vol. xlix. or in the Hift. and Philof. of Earthq. 

Sect. 



\ 






>* 



I 

\ 

1 

\ 

p 



{ 



\ 



i 




I 




I 



t 



' 




13 




Sec t. IL 



23. How well foe\ 



thefe fads may agree with 



tshe fuppofition before laid down, 
fires are the caufc of earthquakes 
ever, may 



That fubt 



doubt, how- 
it is poffible 



that fires fhould fubfift, which have no communica 




remain ; vtz. 



how 



tion with the 



utward air ? In anfwer to this 



I 



might alledge the example of green plants, which take 



by fermentation, when laid together in 



heap 



fire -J - . -err 

where the admijGTion of the outward air is fo far from 
being neceffary, that it will efFedually prevent their 



doing fo 



t-H 



But 



pafs by 



we have many 




ilances more immediately to the purpofe. , 

24. It can hardly be fuppofed, that the fires of the 
' erality of voleanos receive any fupply of frefli air 
(for this muft effeaually be prevented by that vapour 
which is continually rufhing out at all their vents), 

and yet they fubfift, and fiequently even increafe, for. 

* ' fires of the very fame 



many ag 



Now, thefe 



kind w^ith thofe, which I fuppofe to be the caufe 



©f earthquakes 
the purpofe 



Other fadS) ftill more exprefly 
follow 



5. In the earthquake of the ift of November 
755, we are told, that both fmoke and light flames 



and 



were feen on the coaft of Portugal, near Colares 

that, upon occafion of fome of the fucceeding fhocks, 

a flight fmell of fulphur was perceived to accompany 

foe, which came from the fea, from the fame 



s. 



a 

C4 



quarter, whence the fmoke appeared 



^ 3> 



^ ^ 



* SeePhilof. Tranf. vol.xlix. p. 414 




26 





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14 

6. In an account of an earthquake in New Eng 

land, it is faid, that at Newbury, forty miles from 
Bofton, the earth opened, and threw up feveral cart- 
loads of fand and aflies ; and that the fand was alfo 

fulphur, eniitting a 'blue 




(lightly impregnated 
flame, when laid on burning; coals *. 
27. One of th 



maica 



6o 



has thefe words 



of the earthquake in J 



In Port-Royal 



<C 



cc 



«c 



and in many places all over the illand, much ful 



ph 



combuftible matter hath been found 



1 - 



cc 



cc 



(fuppofed to have been thrown out upon the 
opening of the earth), which, upon the iirfl touch 

of fire, would flame and burn like a candle. 



8 



cc 



St. Chrifliopher 



was heretofore much 



cc 



cc 



cc 



cc 



troubled with earthquakes, which, upon the erup- 
tion there of a great mountain of combuftible mat- 
ter, which fl:ill continues, wholly ceafed, andjiave 



been. felt there fince 4- 



ip- Ag 



v/e 



told, that 



the 2oth No 



vember 
the fea. 




burning J ifland was raifed out of 



Tercera 



of the .Azores 



which 




feveral houfes were (haken down by an earth 



quake 



which attended the 



Pt 



Of it. 



Th 



ifland was about three leagues m diameter, -and nearly 
round ; from whence it is manifdl, that the quan- 
tity of pumice fl:ones and melted matter, which mufl: 
have been requifite to form it, was amazingly great : 



* See Philof. Tranf. N^ 409 
Eames's Abr. 



part ii. p. 20U 



4-. See Philof. Tranf. N** 209. or vol. ii. p. 418. Lowthorp*s 



Abr. 

t See Philof. Tranf. N^372 
Eaaie&*3 Abr, 



part lie p. 203 



m 



t 






i 



-fc 



1 1 



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-=- J" J 



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h .' 



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t 



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-fc 





15 

m all proBabillty, it muft have far exceeded all that 
has been thrown out of -^tna and Vefuvius together 
within the laft two thoufand years. This may ferve 
to fatisfy us/ that the fire which occafioned all this, 
mufl have fubfifted for many years^ not to fay ages, 
and this without any communication with the exter- 
nal air. It is worth obferving, that * feveral in- 
ftances of this kind have happened amongft the 
Azores. There are befides many marks of fubter 

raneous fires about thefe iflands, feveral places fend- 
ing up fmoke or flames. Thefe iflands are alfo fu^ 

jedl to violent and fi-equent earthquakes*. 

3 pi We have more inft:ances to the fame purpofe, 

near the iflknd of Santerini in the Archipelago, where 
there have been feveral little iflands raifed out of the 

fea by a- fubmarine volcano. The eruption of one of 

thefe in the year 1708, with all the circumilances 



i_- 



that attended it, we have a very good account of in 



the *!» Philofophical Tranfad:ionsi It was raifed in a 
place where the fea had been formerly 100 fathoms 
deep, and was attended with earthquakes before it 

file wed itfelf above water^ as well as after. It is re- 



portedi that the ifland of Santerini itfelf was origin, 
nally raifed out of the fea in the fame manner ; but^ 
fee that as it will, we have certain accounts of new 
iflands raifed' therCi or additions made to the old ones, , 
from time to time, for above i^oo years backwards^, 
and there have always- been ^rthquakes. at the timcL 
of thefe eruptions,. 



* SceHift. and Philof. of Earthquakes, under the titles .Azores,^ 

Iflands raifed, ^c. 

t See N** 314, 3179 and 33a. cr vol»v. part ii. p. 19^. Jones's 
Abr. 

ai. Anr- 



L'. 



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'. in: 



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year 



[. Ariother example of the fame kind happenei 

Manila, one of the Philippine iilands, in the 

This alfo was attended with violent 
to which that ifland, as well as the reft 



IS^ 



thquakes 
of the Philipp 



very much fubje£t 



3 2. We may add to thefe, the many inflances of 
vaft quantities of f pumice ftones, which have beea 
fometimes found floating upon the fea, at fo great a 
diftance from the ihore, as well as from any known 
volcano, that there can be litde doubt of their being 
thrown up by fires fubfifting under the bottom of the 



ocean. 



with 



g 



33. From thefe inftances^ we may 
probability, conclude, that the fires of volcanos pr 



duce earthquakes 



I do not, however, fuppofe, that 



the 



hquakes, which are frequently felt in the 



the fires 



neighbourhood of volcanos, are owing ^ ^ 

of thofe volcanos themfelves 5 for vokatios, giving 

pafTage to the vapours that are there formed, fhould 



rather prevent them, as in the inftance at St. Chri 




34 



bcfo 

We alfo meet with freq 



#. 



nftances con- 



firming the fame thing amongft the Andes. Antonio 
d'Ulloa (fpeaking of what happens amongft thefe 
mountains) fays, " Experience ftiews us, that, upon 
" the frefh breaking out "of any volcano, it occafions 
« fo violent a fhock to the earth, that all the villages, 

^< which are near it, are overthrown and deftroyed. 




/. 




See Philof. Tranf. vol.xlix. P. 4^59 



t See Philof. Tranf. N"* 372. or vol. vj. part ii, p. 204* »"« 

3^*402. or vol. m part ii. p. 43- E»n»«^« ^°^» 



^c 



as 



6 



1 



-) 



■^ 







I 






€C 



cc 



cc 






th 



dinth 
This {hoc 






of the moiintaia 



* 



Car 



which 



may, witnout 



leaft impropriety, call an earthq 

accompany the erupti 



ke, 



i 



*' dom found to 



after an 



cc 



cc 



cc 



<c 



opening is once made ; 

is perceived, it is ve 

after the volcano has once found 



or 

y 



if fome fm 

fideral 



t! 



fo that, 



TO 



nt, 



a 






ceafe 



ftandii 



2 



" to be on fire 



The 





the matter of it cont|naes 
reater earthquakes, there^ 



S 



fore, feem rather to be occafioned by otlier^ fires, th 
lie deeper in the fame trad of country 



erup 



% 



of volcanos, which happ 



at the fame 



with earthquakes, may 
afcribed to thofe earthq 



wi 



more probability, be 
than the earthquakes 



the eruptio 



whenever, at leaft, the earthq 



of any confiderable extent. 
fufficiently manifeft at prefent 



better underfto 




ply 



po 



/ 



fX 



what will be faid hereafter concerning 



If this don't appear 
it will, perhaps, be 
to the prefent pur- 
local 



thquakes. 

/ _ 

* It does n^T^pear altogether certain, from the expreffion 

madeufeof in the French tranflation ^^^^ "^^'^'^ lX"flt^^. 
this), that Carguayrafo might not have been a V^lcan^^ "^^^ 
times, which it afierted to have been the cafe by ^Y^-^fTtur 

mine. It is poffible alfo, that the fame "^/y^^. ^'J.?,^^ t Innv^ 
mentioned in the next article ; and, indeed, it .s difficult to know 
it to be othervvife, in any inftance, among the Andes, where the 
volcanos are generally foind at inacceffible he^^ts. But al lowmg, 
that all thefe were only old volcanos, which broke out afrelh yet 
thev will ferve at leaft to fwdl the number of them m the fame 
neighbourhood, as well as to fhew us, that there may, very pro- 
bSwv be many more, which lie hid : for thefe ihewed no marks 
o t 'e'irtrfl'L, tin: by their eruption they melted a vaft quan- 
tity of fnow, with which they were before covered, ^nd wh.ch 
being reduced to water, did great damage, by overflowing the 
country round about* 



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in 

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ill 



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iijilli! 

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Sect. III. 

35". It may be ajfked, perhap 
fuppofe, that feveral fubterraneou 



fi 




we ihould 
exift in the 



g 



hbourhood of 



In evidence of 



\ 



we have frequent inflances of new volcanos breakinc 
out in the neighbourhood of old ones : Carguayrafo 
juft mentioned, may fupply us with one example t( 
this purpofei and, in the night of the 28th of Odio- 
ber 1746, in which Lima and Callao were deilroyed 
no lefs than four * new ones burfl forth in the ad- 
jacent mountains. 

^6, To the fame purpofe, we may alleg 
fiances of many volcanos lying together in the 



tra6t of country 



cc 



fo few as forty 



for example, the many place: 

gft the Azores, which 



either do now or have formerly fent forth fmoke and 

ilames j the many volcanos alfo amongft the Andes, 

already mentioned : thus i^tna, Strombolo, and Ve- 

too, are all in the fame 



fuvius, I may add Solfatara 



neighbourhood : and Monf. Condamine fays, 

traced -f* lavas, exactly like thofe of Vefuvius, al 
way from Florence to Naples. In :}; Iceland alfo 



heh 



as 



the 

we 



h 



befides Haecla 



only feveral other 



^ 



but alfo a great number of pi 



y 



that fend up ful 



V- 



* See d^UlIoa's Voyage to Peru, part ii. book L chap. 7. 

t See Phil. Tranf. vol. xlix. p. 624. All thefe lavas, as well 
as the volcanos juft mentioned, lie in a continued line. The fame 
thing holds good in the volcanos of the Andes alfo. This is a fadl 
I muft defire the reader to attend to, as it ferves to confirm a very 
material do£trine, which I (hall have occafion to mention here- 
after. See art. 44, 45, and 46. 

t See Horrebow's Natural Hiftory gf Iceland, 

phureous 



-\ 



\ 



|<iar-;v 



\ 



/ 



r 



1 




/ 



1. 









phureous vapour 



But the examples of this kind 



are few inflances 
duce? of" ^fm Se Volcanos, without evlde 



fo frequent, that^thei 



b 



pr 



mark 



either that there have been others formerly 



5 



hbourhood 



or that there are, at prefent, fub 



•^7 



^_ fires near them. 

This frequency of fubterraneous, fires 



the 



hbourhood of 



will ap|: 



ilill more 



neighbour J:]Ooa or voicanub, wm a^|;wc.. ^w... ...^^ 

probable, if we confider the internal Ih-udure ot t[ 



th 



and, as it will be necefiary alfo 



der 



underftand what follows, to know a little more o 
this matter, than what falls under common obferva 




ive the reader fome 




tion, I ihall endeavour to 

count of it. . 1 r 

.. The earth then (as far as one can judge from 

the"* appearances), is not compofed of heaps of matter 

cafually thrown together, but of regular and uniform 

ftrata. Thefe ftrata, though they frequently do not 

exceed a few feet, or perhaps a few inches, in thick- 

sn extend in length and breadth for 



\ 



many 
nefs ( 



yet 



oft 



mile 



and this without vary 



the 



thick 



fiderably 



The fame flratum alfo preferve 



1 



L uniform charader throughout, though the ftrata 
mmediately next to each other are very often totally 



/ 



differ 



Thus, for infla 



we fhall have, pe 



^ 



hap 



ftratum of potters clay j above th 



flra 



of coal : then another flratum of fome other kind 



of clay 5 next, a 



fliarp 




rit fand ftone 



again 



again 



fand ftone 



again j 




, next, perhaps; 

above that; and it frequently hap p 
of thefe exceed a few yards in thieknefs 



then clav 
and coal 



that 



Th 



are, however, mai 
kind of matter is 



y 



nftances, in which the fame 



e^ 



A. 



inded to 

C 2 



depth of fom( 

hundred 



r 



r 



s 









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L I 

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*:.; 






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r" 






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'^ !' Ill . 



• I K 



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t 



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l)i[ 



^ 









'L L 

i; 1 



■ ■ iii'i 

i i W ! 

IS!!'' 









■',! ;!' ' 



mi 



I ! 11111 1 



M'j' 



ii 



i i !" ill 



)• 






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El 






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4 P 






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--^ 




20 




mafi 



dreds of yards ; but in all thefe, a very few only 
pted, 




of each is not on 



but 



tne 

is again fubdivided into a 




mtinued 
number 



of thin laminsE, that feldom are more 



three feet thick, and frequently not fo much. 
39. Beiide the horizontal divifion of the earth into 

thefe flrata are again divided and fhattered by 



many perpendicular filH 

few and narrow, but oftentimes many, and of 



which are in fome pi 



liderable width. The 



are 



alfo 



■t 

wh 



e a particu 



1 



ftr 



any 



inltances 



fliall have almoft no 



fiffures at all, though the flrata both above and be 



ow it are confiderably broke 



th 



happens fre 



quently in clay, probably on account of the foftnefs 
of it, which may have made it yield to the preiiure 
of the fuperincumbent matter, and fill up thofe fif- 
fures which it originally had ; for we fometimes meet 

correfpondent 



/ 



th inftances in mines, 



wh 



ere 



the 



fiffures in an upper and lower flratum are interrupted 

in an intermediate flratum compofed of clay," or fome 
fach foft matter. 



40. Though thefe fiffures do fometimes correfpond 



another in the upper and lower flrata, yet this 



is noi generally the cafe, at leafl not to any great di 
fiance: thofe clefts, however, in which the large 



of metals are found 



tion to this obfei 



excep 



through many fl 

depth 



for they fometimes pafs 



d thofe of different kind 



to 



4 



, From this conflitution of the earth, viz. the 
of correfpondence in the fiffures of the upper 



d lov/er fir 



of thofe fl 



hich are little or not at all fhattered, it will come to 



pa 



n 



' ■ M*».. V^^^ 



-r ■'W % 



.- ■ .--r^v ' '/ 't 



■»ci'-v .*»*^ 



\ 




■> " . 



H ^ ' ' -" 




f 







* . 




I 



\ 




21 




pafs, tliat the earth cannot 
direction * perpendicular to the hoi 



eafily be feparated 



in a 



if we take 



any confiderabie portK 
horizontal an 



geth 



but in 



the 



dhefion 



between one ftratum and another, it may be feparated 



without diffi 



4 



Thofe fiifures which are at fome depth b 



1 



ow 



the 



furface of 



earth, are 




liy found full of 



but all thofe that are below the level of 



v^ater; _— - • r i, 

fea, muft always be fo, either from the oozing ot th 

fea, or 



rather of the land waters between the ftr 



43 



I 



e 



R 



of the earth are frequently very 



much bent 



beir 




raifed in f 



plac 



d de 



prefTed 



in 



ithers, and this fo 



with 



very 



quick afcent or defc 



but as thefe afcents and de 



fcents 
if we 



great meafure, compenfate one anothe 



lar 




f country 





ther, we may 

early h 




look upon the whole fet of flrata, as 
rizontally. What is very remarkable, h 

their fituation, is, that from moft, if no 

ads of high and mountainous countries, the flii 







r, m 

large 



th 



he in a fituation more incUned to the ho 

the country itfelf, the f mountainous countries being 

generally, 



* What I fiiid before of thofe deep clefts, in which metals are 
found, will not affea this conclufion ; for tbev are confiderably 
different from either perpendicular or plane feitions of earth ; they 
are frequentlv interiupted by (irata of clay, or other foft matter; 
and they are, in moil parts, either filled up with rubbini, or with 
ores and fpars, that adhere as firmly to the rocks on both fides, as 
if they com.-ofed one,c<)ntnued ftratum with them. 

f It fee.T.s very probable, from many appearances, not only that 
the jnountain, ous counmes are formed out of the lower flrata of the 

earth, but thac fomeumes the highefl hills in them are formed 

out 



.-\ 



1^ 



*; 



( 



1 



I r 



I 1. 



y 



I L 



'fc 



% 



^ 



f 



f 



i 



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r 



i 



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4 



4 





■ n 

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L 



I 




p t 



r^ 



■ I) t * 

ilii 



I'll ' ' 

m ■[ 



hi!! 

r P ;" 

r 
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\ 



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i4 



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: i t 






I 



■■if'i 



:i 






- t. 



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1 III!!: : 









tr 



:l!i':'|M 

Hi" i ' 



I ^■ I 






■, ;' 






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ii 



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;6 

■ ill'' 



I'.. Ill 



^'l' 



^ --^ 





\ 




22 




lly, if 



alway 



formed 



of the lower 



ilrata of earth. This fituation of the ftrata may be 



not 



dy reprefented in the followin 




manner 



Let a number of leaves of paper, of feveral different 
:s or colours, be pafled upon one another ; then 



forts 

bending them up 




ether into a ridge in the middle 



them to be reduced again to a level furface 



by a plane fo paffing through 

le part that had been raifed 



t 



raifed 



off all 
middle now be 

and this will be a good general 



1 



eprefentation of moil^ 
mountainous countries, 



if 



of 



together 



11, large trads of 
th the parts ad- 



jacent, throughout the whole world 



44. From this formation of the earth 



it will fol- 
low, 'that v/e ought to meet with the fame kinds of 
earths, ftones, and minerals, appearing at the furface 



g narrow flips, and 



a 



parallel to the greatefl 



ife of any lor 
ve find them 




and fo 

The Andes in South Amer 



ridges of mountan 



has been faid before, have a chain of 
extend in length ab 



5000 



miles 



faa, 

as it 
i^olcanos, that 

hefe volcanos. 



in all probability, are all derived from the f fame 



out of ftrata ftill lower than the reft, which, perhaps, may always 
be the cafe, where they have volcanos in them. [See a repre- 
r-_*v,*: c ^v,;. ir. fK*. Platp Ficr. -2.] In other inftances, how- 



ever 



.V.., it often happens, that the hills, to which thefe high lands 
ferve as a bafe, are not only formed cut of the ftrata next above 
them, but they ftand, as it were, in a diih, as if they had depreflbd 
the ground, on which they reft, by their weight. , . . , 

* Fio-. r. reprefents a feaion of a fett of ftrata, lymg m the 
fituation juft defcrlbed : the feaion is fuppofed to be made at right 
andes to the length of the ridge, and perpendicular to the ho- 

— 



rizon. 



36 and 53 



F 



5 



ftratum 



i:V V', 



.^^'-^v * '/'', 



A** 



i:\: M'm^'^ 




\ 



« 



■! 



I 



/ 



- -'\^'- 





>. 



V 



♦ 



/ .. 



I 



/ 






ftratum. Parallel to the Andes, is the S 



her 



loiw ridse of mountains, that run between the Andes 

: and " thefe tv/o ridses of mountains run 



<c 



d the fea 
within fig 



r 



one 



anoth 



& 



er. 



and 



noft equa 



> 



be 




" for above a thoufand leagues together *," 
each, at a medium, about twenty leagues wide, 
gold and filver mines wrought by the Spaniard 
found in a trad of country parallel to the diredtion of 

through a great part of the length 



The 
, are 



thefe, and 



tendin 



of them 




45-. The fame thing is found to obtain in North 



America alfo. 
the river St. L 



Theg 



which 




fe 



lurence, are kept up by a long ridge of 
mountains, that run nearly parallel to the eaftern coaft 
In defcending from thefe towards the fea 
fets -f- of flrata, and in the fame order 



fame 



g 



^=i 



rally 



met with throughout the greateil part of their length 



^ 



46. In Great Britain, we have another inflance 

the fame purpofe, where the diredlion of the ridg 




in 




about a point from due north and fouth, 

nearly from f N. by E. to S. by W. There are many 
more inftances of this to be met with in the v/orld, if 



we may jud 



from circumftances, which make 



highly probable, that it obtains 




number of 




and in feveral they feem to put it almoft 



of doubt 



47 



The reader 



any infl 



is not to fuppofcj however, that^ 
the higheft rife of the ridge, and 



r 

* See Acofta's Natural Hiftory of the Indies. 
f See Lewis Evans's Map and Account of North America. 
j Of ihis I could give many undoubted proofs, if it vi^ould not 
loo far exceed the limits of my prefent defign, and which, for that 

reafon, I am obliged to omit. * 

the 



1 



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Tfcl 



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r 



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. 4 



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l! il 

■ I m 

lilii 

!! ill 

•'III! 



'I'l 



II 



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Ml 



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r 'W ! 






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if 



fii:i.;^i If 



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.:ii 



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iif!!:ii'i 

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li-yi 







frf 



24 

of the ftrata from 
^ach fide, is perfectly uniform ; for they 



quently very 



fiderable inequalitie 



qualities are fometimes fo 




for fome fmall dift 



and thefe 
that the flrata are 

the contrary way 



f r o in 



This often 
akes it diificult to trace the appearance I have been 



the general inclination of them 



relating, 



which. 



knowlege of the 



foffil bodies of a large trad of country, it is 
^ofiible to do. ' 

48. At confiderable diftances from large ri 

mountains, the flrata, 



hardly 



d 




of 



for the moft part, alTu 



early level 



d 



Uy formed out of the lower flrata, fo 
the more" level countries are generally formed out of 



thi 
fel 



t upper ftrata of the earth 

49. Hence it comes to pafs, th 



m 



of 



kind 



fame flrata are found to extend them 



way 



as well 



countries o 



fE 



jR:ance of this 

id and F 



n 




ption of the Channel, and 



breadth as in length : 

the chalky and flinty 

ce, which (excepting 

the clays, fands. 



&c. of a few counties) compofe a trad of about thr 



hundred miles each way 



SO- 



Befides the raiiins: of the fli 




m a 



ridse, 




flruclure of 



her very remarkable app 



the 



though a very common 



and this is what is ufually called by miners. 



the 



P 



m 




down of the ftrata : that is, the 



wtiole fet 



/ 



of llrata on one fide a cleft are funk dov/n below the 



If: 

ilr 



of the correfpondin 




ilr 



on 



the 



fi 



fome cafe 



difference in the level of the 



on 



different fides of the cleft, fhould be 

very 






t 



f ' 



y- 



i 



ll . «-- - 



. -3-.d 





*** 







* 



I 



k 






' 






i 





very c 



25 
onfiderable, it miy have a great cEcd in pro 



diicing fome of the fingularities of particular earth- 
quakes 








R 




IL 



3 




N the former part of th 



eiTay 



I have re- 



unted fome of the principal appearances of 



rthquak 



thofe particulars in the ft 



quakes, as v/eu as uiuic paitiu^iax. ... — :"-~ 
of the earth, upon which I fuppofe the e ap- 

From what has been already 



th 



fom 



pearances to depend. 

faid, I think it is fufficiently manifeft 

inftances at leaft, earthquakes are adually produced 



now, therefor 



byfubterraneous fire,, 

be ihewn, how all the appearances above^ recited 



many other 



minuter circumftances attending 



hquakes 



may be accounted for from the fame 



caufe 



I - 



S 



I 



f2 



The returns of earthquakes in the fame pi 



either at fmall or 



large 



intervals of 



are 



y 



fiftent with the caufe affigned : fubterraneous fir 



from their analogy to volcanos, might reafonably be 

' gh we had not 



fuppofed to fubfift for many 




thofe 



ftances f already mentioned, which put the 



X 




* 



Fig. 2. 



(< 



Ti- 



the manner juil defcribed. 
perpendicularlv to the horizon, and at rigl 
of the cleft : an inftance of .this kind, am 



at angles to the diredion 



Mend 



H: 



amongft the coal miners of 

mentioned in the Philof. Tranf. See 



tthe account of it, together with a drawing, in M 360, or J 



Abr. vol.iv. partii. p;. 260. 

f See art. 28 to 32 ' 



D 



« 



matte; 



\ 



\ 



w 

i 
-V I 

w 

i 



i 



i - 



F 




-< 



[i 



X 

? 



f. 






V 










^ -** 



J- r- 





^ 



J 



I 

* 
I 

h 
I* 1 



'1 \ 



V 



r f 



p I 

I 




' liiP il i 



'11 



k I ■t\'t -^ ' 



^ |ii I i ■ 

' h u I ■ ' 

1 ifi:!! U 
i '-ill' I: ■ 

'H -I 

^ IPl -kl J^ - 









! ; if I 



[ 



■''III 
■M 



' it 






t r ' 



;^' 



: F 







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I I 



^ 



■i 

i 



;+ 



/ 





matter out of doubt 



26 



And, as it frequently happens 




that volcanos rage for a time, and then are quiet 
again for a number of years ; fo we fee earthquakes 
alfo frequently repeated for fome fmall time, and 

pting, per- 
now and then fome flight ihock. And this 
gy between earthquakes, and the efFeds of vol 



then cealing again for a long term 



hap 



canos, is fo gi'eat, that I think it cannot but appear 



flriking to any 
Both 



who will read the 



of 



and compare them together. The raging of 

volcanos is not one continued and uniform effed : 



but 



eifedl, that is 



peated at unequal 



and with unequal degrees of force : thus, for in- 
ilance, we fhall have, perhaps, two or three blafls 

difcharged from a volcano, fucceeding one another 
^t the interval of a few feconds only : lometimes the 
intervalsare of a quarter of an hour, an hour, a day, or 

perhaps feveral days. And as thefe intervals are very 



qual, fo is the violence of the blafts alfo 



fome 



times ilones, &c. are thrown, by thefe blafts, to the 



diilance of fome mi 



ther times 




not 



the diftance of a hundred yards. The fame dif- 

intervals and violence of 



ference 



is obferved in the 



the fhocks of earthquakes, wliich are repeated at 



fmall intervals fon fonle time 



d' 



.^^'^il! 



ip 






111 'it 



.■•\- 






t 

y h 



■'l^r ^ 



I'm I 



:it*Mi 



II 



^ ri 






\ 






k I 



. \. 



^■If! 



'> t 



53 




g 



Sect. IL 

frequency of earthquakes in the 

ftrong 



mountains, is a 



ghbourhood of 
gument of their proceeding from a caufe of th 



burn in ^ 



e 



fame kind : and the analogy of feveral volcanos lying 
together in the fame trad: of country, as well as new 



breaking out in the neighbourhood of old 



I 

tends 



/" 






> 



1 l^''^' 






• .<. 



■ ! "i 



^i 






\ 
\ 






y 



.'-, ' ^ /■ .^-^v 



* %< 



u^M^tfOL^X. 




--- tj 







/ 






I 






tends greatly to confirm this opinion 3 but what makes 



ftill the more probable 



peculiarity in the 




^\ 



jftrudture of the earth, ah'eady mentioned 



I obferved 



before, that the fame ftrata 




Uy very 



nd that they commonly lie more inclining fi 



live, { 

the mountainous 



fel 



than the countries them- 
s : thefe circumflances make it very probable, 
thofe * ftrata of combuflible materials, v/hich 

break 



Th 



argument 



* It has been imagined by fome authors, that volcanos are pro- 
duced by the pyrites of veins, and that they do not owe their origin 

to the matter of ftrata. In order to prove this, it is alleged, that 
volcanos are generally found on the tops of mountains, and that 
thofe are the places in which veins of pyrites are generally lodged. 

being taken from obfervations that have their 
foundation in nature, ought not to go unaftfwered. In the firfl 
place, then, the pyrites of veins, or fifTures, are not found in fuf- 
ficient quantities, or extending to a fufficient breadth, to be fup- 

pofed capable of producing the fires of volcanos : it very rarely 
happens, that we meet with a vein or fiffure five ©r fix yards wide 5 

and when we meet with fuch an one, yet, perhaps, not a twentieth 

part of it at moft fhall be filled with pyrites j but the fires of vol- 
canos, inftead of being long and narrow, as if the matter that fup- 
plied them was depofited in veins, are generally round, and of far 



greater breadth than veins can be fuppofed to be. 



Co 



Monf. ^ . 
at this time, five 



or fix hundred fathoms wide ; [fee Hid. and Phi 

p. 195] and the burning ifland that was raifed out of the fea near 



Terce 

and nearly round. 



] 



Befides this, it is very difficult to conceive how any matters 
lodged in veins can ever take fire; for, excepting where the veins 
are extremely narrow, they are almoft always drowned in a very 
great quantity of water, which has free accefs to every part of 
them : neither are the pyrites of veins, by any means, fo apt to 
take fire of themfelves, as thofe of flrata ; and if, indeed, there are 
any of them that will do fo, yet they are but few in comparifon of 
thofe which will not : all thofe, which, befide iron and fulphur^ 

9j)ntain copper, or arfenic, even in a very fmall proportion, are not 



P 2 



at 




I. 






I 




\ ■ 







4 






n 



P 




I 



i 



.H 



■--^. 




r-" 




1 I 







' 



■ - t] 



iii'i 



1 



I,. 



fi; 






\i 





28 

break out in volcanos on the tops of the hills, are to be 
found at a conliderable depth under ground in the level 
and low countries near them. If this fhould be the cafe, 



"\ 



iV 



y 




w ^ 



* 



'k 



! ) 



^ 



' V u 



^ ^ 



"T r 



. hh 



. ii 'i i; 



L J 



iiiir 

1 . 'I I 

■ I h- 'K V 
»I H I. 

^ ,- 11 ' 



'Ml 

i I J i I 

Ml' 










- ,: . Hi 1 



h ■ I 



iiiii 



► J 









■ i| 



.1 



I 



■ r 



■pi' 

M 

jiliiiiiii 



-^ 



b I 



!■■■'[!■ 



.1 I. 



III 



•iKiiti; 






^ I' t ■ 



at all fubjeft to inflame of themfelves. On the other hand, moft of 
the pyrites of ftrata, if not all of thenr, have this property more or 
lefs. There are alfo two forts of ftrata, in which pyrites are 



lodged in the greateft abundance, that have the fame property. 



-^ 



/ 



and that frQC[uently in as great a degree as themfelves : thefe are 
coals and aluminous earths, or {hale. There are fome kinds of 
both thefe, that, upon being expofed to the external air for a few 
months, will take fire of themfelves, and burn. Thefe two forts 
of ftrata are alfo near akin to each other; they are generally found: 
to accompany each others they are both of them generally inter- 
mixed with, or accompanied by ftrata of iron ore; and they both 

of them, for the moft part, either contain, or are lodged amongft,. 
the remains of vegetable bodies; and thefe remains of vegetable 
bodies, in the aluminous earths, are frequently either wholly, or 
in part, converted into pyrites, or coal, or both. Numberlefs, in- 

-ftances of this are to be met with in the aluminous {hale of Whitby 
iand other places. 

i It is very probable, that to fome ftratum of this kind the fires of 
volcanos are owing ; and this feems to be confirmed by the fimi- 
larity of the materi-als, which are thrown up or fublimated by the 
fires of volcanos, to the matter of the aluminous earths. Solfatara 
produces fulphur, alum, and fal ammoniac. The two former of thefe 
are very eafily to be obtained from the aluminous earths, and, I 
fuppofe,. the latter alfo; at leaft it is procurable from the foot of 
common folSl coals, and probably, therefore, from the foot of that 
coaly matter which is intermixed with fuch earths. 

The aluminous earths, moreover, not only have feveral ftrata of 
iron ore lying in them, but they alfo contain a confiderable propor- 
tion of iron in their compofition. In correfpondence to this, we find 
the lavas of volcanos, and other mattejrs thrown out from thence^- 
frequently containing a great deal of iron, the fmall duft of them- 
readily adhering to the magnet. 

As to the pyrites of veins, I much doubt whether they ever con- 
tain alum, or fal ammoniac; at leaft they are very rarely found to 
«;ontain either the one or the other^ 




.VK.ir \ 



■ ..-^-^v 



V^:.^.v^Y'- 



w:*SCi.^v mim^ 



I 



4 



./ 



\ 



■ V---- 




Hi 



k 



i 



^\ 




J 



1 



^ 



»V 



- \ 






./. 



\t 





I 




and if tlie fame * ftrata fliould be on fire In any places 
under fuch countries, as well as on the tops of the hills, 

of whatfoever kind, raifed from thefe 



all 



pours 



fires, muft be pent 



nlefs fo far as they can open 

whereas the 



themfelves a paffage between the ftrata ; 

Dours raifed from volcanos find a vent, and are dil- 



charged in blafls from the mouths of them 



if, when they find fuch 



they are yet capable 



of iliaking the country to the diftance of ten or twenty 
miles round, what may we not exped from them 



■when they are confined 



We may form fome id 



of the force and quantity of thefe vapours from their 
rfeds : it is no uncommon thing to fee them throw 
-up at once, fuch clouds of fand, aflies, and pumice 



\ 



ftones, as 



^.able of darkening the whole air, 
d covering the ncighbourin 

miles diftance 




:itry with a fho 



of duft, &c. to fome 

alfo. 




rei 



fc 



of fome tons weight, are often thrown 



difl 



of 



and Monf. B 



v-^ 



or three miles by thefe explofions -. 

,^1- tpll« nc ■ that he met with ftones 




iked 



:„ ;^eZ« =d ';'<;;'7ai'e Iri .hroughout .he whole extent 

cf Yt? In anta to this, it may he faid, that the fame ftratum 
may differ a little in the richnefs of its combuftible prmc.ples m 

..J . i" _. -.- «,^-v^..c fV>^ frpniipnrv of the fiflures, either 



uency 



ditrerent places ; or, pciu^^po, tiiw .i^.^--^*-/ ^- " -^ ' i«-. 

b the combaftlble ilratum itfelf, or the ftratum next to it, may let 
n fo much water, as to prevent its takmg fire excepting m a few 
daces%ut, if this once happens, the fire will not eafily be put out 
acrain but i will fpread itfelf, notwithftanding the fiffures that lie in 
kf way though they are filled with water j for the matter on fire 
wiirbe^'in fome degree at leaft, in a fluid ftate ; and, for this reafon,. 
U mu ft neXily Ixpel the ;ater from the fiffures, l^oth on ac- 
count of the extenfion of its own dimenfions by the heat, and of 
the weight of the fuperincumbent earth, which, preffing it, wUi 
make it fpread laterally. ' : 






h 

fcf. 



"t 



^. 



if 



■X 







i 



■1 1 



m^. 



\ 



V 



:a^i> 



' » 



-^^— ■^- 



CUj^ 




\ \ 





1 



V 






ii 



r 
I 

\ 



i 



li 



I rli F j! 

!H 1 




1 I I 



>, I 



■ ' IT '" 



I 



;■! 



m 






!^;i!i'i 



f\ 



MP ' ' 



■'Jill 






rt 



1 



m ; 






■m ' 






iifesii" ' 









.% 



.!i 



'■'ill 



•K 



1 



n III 4^1 



f I F 



Mtllr 






ii' 



f 



m 




■'h' 









■'l!.:il ! ; 



m' ' I I 



f4 ! 

■IS''!! ! 



1_ 



3n South America 




of 



30 

g 




or nine feet diameter 



had been thrown from the volcano Cotopaxi, by 



one of thefe blafts, to the difla 



three leag 

54. If we fuppofe 



of more than 



/ 



thefc 



the caufe of earthq 



P 



when pent 



we mufl naturally 



"P 

expedt, from what has been jufl faid, that the moft 

extenfive earthquakes fhould take their rife from the 

level and low countries ; but more efpecially from the 

fea. which is nothing elfe than waters covering fjch 



countries. 



Accordingly we find, that the great earth 




quake of the ift November 175-5, which was felt 

'aces near three thpufand miles diftant from each 
other, took its rife from under the fea; this is mani- 
feft, from that wave which accompanied it, as (hall 
be fhewn hereafter. The fame thing is to be under- 
flood of the earthquake that deftroyed Lima in the 

year 1 744 which, it has been faid, was felt as far as 
Jamaica ; and, as it was more violent than the Lifbon 



rthquake, fo, if this be 



muft 



pro 



babihty, have been more extenfive alfo. There have 
been many other very extenfive earthquakes in South 
America : Acoila fays, that they have been often 

two, or three hun 



wn to extend themfelves 



dred, and fome even five hundred leagues, along the 



coaft 



Thefe have been generally, if not always 



tended with waves from the feaj hut any minuter 



T 



W 



* See Hift. and Philof. of Earthq. p. 195. Don Antonio d'Ulloa, 
an author of great veracity, fpeaking of the fame thing, fays, tliat 
" the whole plain [near Latacunga] is full of large pieces oV rocks, 
^« fome of them thrown from the volcano Cotopaxi, by one of its 
^« .eruptions, to the diftance.of five leagues." See his Voya<ye to 

Peru, part i. book vi. chap. .1. 

-circum- 



y 






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1 
I 






'Tf 



i±h 



i 



I'; 



ti'iisif;:' , 



',P ii T 



.1. ■,.■>. 



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i 



1 



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I 



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y 





31 

circumflances accompanying them are not related 



Indeed it is haidly 



be 



pedted that they (hould 



^ 



be obferved, much lefs that they (hould be related 
when they happened in a country fo thinly inhabited, 
and where one may reafonably fuppofe, that, in ge- 
neral, only the gro&r and more violent efFeds would 
be taken notice of. 



Sect. III. 



55-. I have faid before, that I imagined earth- 
quakes v^ere caufed by vapours raifed from waters 
fuddenly let out upon fubterraneous fires. It is not 
eafy to find any other caufe capable of producing 
fuch fudden and violent effeds, or of raifing fuch an 
amazing quantity of vapour in fo fmall a time. That 
the blalb, difcharged from volcanos, are always pro- 
duced from this caufe, is highly probable 5 that they 

are often fo, cannot admit of the leail doubt. There 
can be no doubt, that confiderable quantities of water 

muft be often let out upon the fires of thefe 



■w 



/ 



and whenever this happens 



wi 



be immediately 



raifed 




the heat of them into a vapour, whofe 



elaflic force is capable of producing the mofl violenf 



effeds 



5:6. Both 



* There are many effeds produced by the vapour of water, 
when intenfely heated, which make it probable, that the force of 
gunpowder is not near eq«al to it. The efFeds of an exceeding 
fmall quantity of water, upon which melted metals are acci- 
dentally poured, are fuch, as, I think, could in no wife be ex- 
pefted from the like quantity of gunpowder. Founders, if they 
are not careful, often experience thefe elFeds to their coft. An 



ed 



Windmill- hill, Moorfield 



" The 
« heafe 







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32 




' f6. Both tlie tremulous and wave-like moti6n ob- 

^rved in earthquakes, may be very well accouoted 

for 




4£ 



h'^at of the metal of the firft gun drove (o much damp into the 

.. mould of the fccond, which was near it, that as foon as the 

*' metal was let into it, it blew up with the greateft violence, tear- 

ine up the around feme feet deep, breaking down the furnace, 

untilina the houfe, killing many fpedlators on the fpot, wid. tne 

ftream?of melted metal, and fcalding many others in a moft mi- 






ferable manner." [See the note at the end of procefs 44th Ox the 
Encxliih tranflation of Cramer's Art of affaying Metals ] 

Other inftances of the violence of vapours raifed from_ water, are 
frequently to be met with: one of Papin's digefters being placed 
between the bars of a grate, where there was a fire, was, atter 
feme time, burft by the violence of the fteam, the fire was a 
blown out of the grate, and a piece of the^djgefter was dnveii 
agarnft the leaf of a ftrong oak table, whxh it broke to pieces 
fSee Philof. Tranf. N° 454- or Martyn's Abr. vol. vuu p. ^^^ ^ 



Wo 



] 

tells 



y 



lis, that h« burft a cannon by the fame means. 

It has been fometimes imagined, that the vapours, which occa- 

fion earthquakes, were of the fame kmd with thofe fulminating 



-camps. 



nftances 



Sis il no th cafe ; it is .me, the force of f.ch vapours is very 
leat, we have had inftances, w.,cre large beams of nmber have 




Tr 



136. or vol. ii 



1 



[f< 



what is this to the force of that vapour, which could throw ftones 



N 



ty ton weigni lu tiic unuaucc ui wi*ww leagues, 

' ' ^ that any vapour, already in the 



\ 



form of a vapour, can, by fuddenly taking fire, mcreafe jts dimen 
fions fo much, as to produce that immenfe quantity of motion, 
which we' obferve in feme earthquakes ; but this is rather to be 
expeded from fome folid body, fuch as water, which is capable of 
bein^ converted, and that almoft inftantly, mto one of the hghteft, 
and perhaps one of the moft elaftic, vapours in the world. Air, 
when heated to the greateft degree that it is capable of receiving 
from the hotteft fires we can make, acquires a degree^of elalticity 

.,-.. _f gjj.. the vapour of 



five times as ereat as that ©t common 



,gu:a- 



/ 






/ 








^ 



Jr, 




33 




for from fuch z 



more 



vaDour. 

X 




^ 

in order to trace a little 
the. manner in which thefe two 

motions 



*" 



A ^ 




t 



V 



y 



mines. 



r 

\ 

gunpowder, whilft it is inflamed, has- al fo about five times the 
elaftic force which it has when cold. [See Robins's excellent 
traa on Gunnery.] Now, if we fuppofe a fulminating damp, of 
any kind, to increafe its elafticity, when inflamed in the fame pro- 
portion, this will be abundantly fufficient to make it produce an}^ 
effefts, which we have ever feen produced by any of the damps of 

^c. And, indeed, whoever carefully examines the effe<Sls, 
either of the damps of mines, or of thofe fulminating damps, that 
are raifed from fome metals, when in fufion, or when they are 
diflblving In acids, will rather be inclined to think, that the force 
of inflamed vapours is fo far from exceeding the proportion of five 
to one, that it falls confiderably (hort of it. 

But though we fhould fuppofe that this proportion holds good, 
where fliall we find a place capable of containing a fufficient quan- 
tity of fuch a vapour, to produce the great efFeas of earthquakes ? 
It will be faid, perhaps, 'in fubterraneous caverns. To this 
may anfwer, that he, who is but moderately acquainted with the 

ftrudure of the earth, and the materials of which it is compofed, 
will be little inclined to allow of any great or extenfive caverns in 

it. But, though this fljould be admitted, how ean it come to pafs 

ihat thefe caverns ffiould not be filled with water ? If it is alleged, 
that the water is expelled, as the vapour is formed, w^hy fhould 
^ot the vapour, as it is fuppofed to be the lighter, be expelled, 
rather than the water, by the fame paffages by which the water is 
to be expelled ? But let us fuppofe this difficulty alfo to be got 
,over, and the water to be removed, and we fhall then have a gage 
for the denfity of the vapour i for it mufl be jufl fufficient to make 
it capable of fuftaining a column of water, whofe height is equal 
io that of the furface of the fea above the bottom of the cavern, in 



/ 



we 



fuppofed 



Now 



^c 



vtimes the weight of water, this vapour mufl be increafed to two 
and half times its original elaflicity, before it can, in any wife, 
jiaife the earth above it ; and if we fuppofe it to be increafed to five 
4imes its original elafticity, it will then be no more than twice abk 
to do (o ; in which cafe, fo much vapour only can be difcharged 

from the cavern^ to produce an earthquake, as is equal to the 



E 



content 



t . 



/ 









I 



s 




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r 

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rri 







■11 ii> 






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b ^^_ ^ ^ J^ ^ bL ^ 





34 




r 

motions will be brought about, let us fuppofe the 






roof over fome fubterraneous fire to fall In. If thi& 
fhould be the cafe, the earth, flones, &c. of which 

r 

it was compofed, would immediately £nk in the 
pielted matter of the fire below : hence all the water 
contained in the fiiTures and cavities of the part fall- 
ing in, would come in contact with the fire, and be 
aimed: infiantly raifed "into vapour. From the firfb 



effoit of this vapour, a cavity would be formed (be- 
tween the melted matter and fuperincumbent earth) 
iiiled with vapour only, before any motion would be 
perceived at the iurface of the earth : this mull ne- 
celfarily happen, on account of the * compreilibility 

. of 



r - 



a 



content of the cavern: and what muft the fee of that cavern 
be, which could contain vapour enough to produce the earthquake 

of the lit of November 17555 in which an extent of earth of nfear 

three thoufand miles diameter was confiderably moved ? or how can 

we fuppofe, that the roof of fuch a cavern, when fo violently 
fiiaken, fliould avoid falling in? eff-"'"^^" " "' ' ' n . , 



eipeciaily,. as it is hardly to be 



fuppofedj that any inflamed vapour whatfoever Ihould be able to 
move the earth over thefe caverns, if they lay at any great depth^, 

fmce the weight of lefs than three miles depth of earth is capable 
of retaining the inPxamed vapour of gunpowder mth'in the original 
dimenfions of the gunpowder itfelf ; and common air, comprefled 
by the fame weight (fuppofing the known law of its compreffion to 
hold fo far)^ would be of greater denfity than water. 

We may afk flill farther, whence fuch vaft quantities of vapour 
-fiiould be formed, or what fouices they mufl be, which would 
not be exhaufted (if they were not again replenillied) by a very few 
^repetitions of fuch immenfe difcharges. 

* The compreilibility and elafticity of the earth, are qualities 
v/hich don^t {how themfelves in any great degree in common in- 
ftances, and therefore are not commonly attended to. On this 
account it is, that few people are aware of the great extent of 
them, or the efFecis that may arife from them, where exceeding 
large quantities of matter are concerned, and where the comprel- 

■ iiv 



^— H^ 



^ 



I 



\ 







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tli';;iii|l 1 



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of all 




35 



kinds of earth, ilones, &c. 
n of the materials imm 



4 



but as the com 




over 



the 



would 



five 



force is Immenfely great. The comprcfiibnity and elaRIcitv 



of the earth may be collected, in fome nieafure, from the vibration 
o^ the walls of houfes, occafioned by the paffing of carriages in the 
ilreets next to them. Another inftance to the fame purpofe, may 
be taken from the vibrations of iteeples, occafioned by the ringing 
of bells, or by gufts of vi^ind : not only fpires are moved very con- 
fiderably by this^means, but even ftrong towers will, fometimes, be 
made to vibrate feveral inches, without any disjointing of the mor- 
tar, or rubbing of the ftones againft one another. Nov/, it is ma- 
nlfeft, that this could not happen, without a confiderable degree of 
compreffibility and elafticity in the materials, of which they are 
compofed : and if fuch fmall things as the weight of fteeples, and 
the motion of bells in them, or a guft of wind, are capable of pro- 
ducing fuch efFecfts, what may we not expe£l fi-om the weight of 
great depths of earth ? There are fome circumftances, which feem 
to make it not altogether improbable, that the form and internal 
firudure of the earth depend, in a great meafure, upon the com- 
preffibility and elaflicity of it. There are feveral things that feeni 

to argue a confiderably greater denfity in the internal, than the ex- 
ternal part of the earth ; and why may not this greater denfity be 

owinff to the compreffion of the internal parts arlfing from the 
weight of the fuperincumbent matter, fmce it is probable, that 
■the matter, of which the earth is compofed, is pretty much 



of the fame kind 



throughout ? 



There is a 




gument for the earth's owing its form, in fome meafur 



ftronger ar- 

to 



the fame caufe j 



u< 



accounts. of the meafures of a degree of the meridian in France, 
Sweden, and America] at the equator, than at the poles, ^ in a 
.greater proportion than it would be on account of the centrifugal 
force, if it v/as of uniform denfity; buty if we fuppofe the earth 
to be of lefs denfity in,3n seguatorial diameter than in the axis, th 
.whole will then be eafily accounted for, from the rifing of the earth 
a little by its elaflicity, the v/e:ght being in part taken off by the 
diurnal rotation : and that the earth is really a little denfer in the 
axis, than in the sequatorial diameter, feems highly probable, from 
;the experiments of pendulums compared with aftronomical obferva- 



jtiojis; 



ived 



E 2 



reconciled 



\ 



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nil 



■ BFT 'ki 



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36 




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would be more than fufficient to make them bear the 

weight of the fuperincumbent matter, this compref- 
fion muft be propagated on account of the elaflicity of 
the earth, in the fame manner as a pulfe is propa- 
gated through the ^ air 
niediately over the c 



and again the materials nn 
ty, refioring themfelves be 



yond their natural bounds, a dilatation will fuceeed to 
the compreffion ; and thefe two following each other 



} 



for fonie time 



1 



ced at the furface of 



bratory motion will be 
earth. If thefe alter- 



and compreffions fhould fuceeed 



iiother at very fmall 



they wou 



like motion in the air, and thereby occafion a con.- 
fiderable noife. The noife that is ufually obferved 



to precede 



mpany 



hquakes, is probably 



owing partly to this caufe, and partly to the 

of the parts of the earth together, occaiioned by th 



wave-like motion before- mention 



that firii 



th the fire, has formed a cavity 



fj. After the water. 



came in contaifl: 

all the reft of the 



water contained in the fiffures, immediately commu 

nicating with the hollow left by the part that fell in 



reconciled with each other, but upon this fuppofition. 



Mac 



Fluxions, art. 681, ^c] It appears, from fome late and 



accurate obfervations 



J"P 



alfo, as well as thofe of the earth, are a little higher than they would 
be, if their, rife was owing to the centrifugal force, and he was of 
uniform denfity ; but if we fuppofe him to be of lefs denfity in the 
equatorial, than the polar regions, then the form may be fuch as 
he would aflume from the refpe£live gravitation of the feveral parts ; 
and any fluid like our ocean, would not overflow the polar parts. 



necefTs 



would follow his general form, as our ocean does that of the 






lar.th.. 







1 






\ 



*■ r 



*% 






J 



V 



\ 





•37 

m out upon the fire, the ileam taking ifs 
From hence may be generated a vaft quantity 
of vapour, the eflFeas of which fhall be confidered 



mull: 




prefently 
iuppofi 




This fteam will continue to be generated 
the fire to be fufficiently g 



fures before-mentioned are evacuated 



the fif- 
the wa- 



beeins to flow very flowly ; when the fteam 



ill be removed b 



th 



;lallicity of 




ready formed v 

the earth, which will ag 

upon the furface of the melted 

lip a little way into all the clefts, by which the water 



fubhde, and, preffi 
matter, will force 



might continue to flow 



By this means, all com 



munkation between the fire and the water 
prevented, excepting at thefe clefts, where the watei 



drippin 



g 



flowly upon the melted matter, 



dually for 




upon 



will g 
that will foon flop 



farther communication .in thefe places likev/ife^ and 

the fifliires, that had been before evacuated, will be 

f the water 



^ 



again 




radually repleniflied by the 




between the flrata 



. As a fmall quantity of vapour aimofl: infl:antly 
generated at fome confiderable depth below the fur 




face of the earth, will prod 



vibratory motion 



fo a very large quantity (whether it be generated at- 
mofl: inftantly, or in any fmall portion of time) will 
produce a wave-like motion. The manner in which 
this wave-like motion will be propagated, may, in 



fome meafure, be reprefented by the following expe 
riment. Suppofe a 



large cloth 



or 



upon 
fut 



a floor) to be raifed at one edg 




denly brought down 
der it, being 

till it 






pet, (fpread 

e, and then 

floor, the air 

rrleans propelled, will pafs 

fing the 

cloth. 



again to the 



ppofite fid 



'^ 



/a 



r 






I" 



.*■■ 
^ 









i 
1 



;? 




— '^ 



'm 



■^ 



!S 







, I 



■L*f| 



r 
r 

I 




v-^^^ 









J I- I 

I T » t 



I 



'■■''• 111" 

L flk -'■ '\ - 



. . 1,1 



1 






■ "K' Hi 



I 



I \ 



t : 






,;VlU 

li 

1l 

•i 









t" hi] ill), 



^Ki]';''i 



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f !■ h 



.i' f 

1 » 



■Hi 



■ ■ r 




.1! 






iiiihiii 



:. ! 



K 






::iF 



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,-■' 



. * 




y t 



Mm 



I J 'H 



I 



It'iHtii:' . 



■1>?l^ 



Tin i 



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■ 'l ! 



I.I, ' 



4 1 



I ' I' I 

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^*Hi;ri ! 



iP.; 



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f Ll |H I, 1 



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'.fii/.i;r 

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ms 



l..^'...,,,,, 

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cloth in a wave all the way as it goes. In like man 

ner, a large quantity of vapour may be conceived t< 
raife the earth in a wave, as it paffes along between th( 



ill 



•ata, w 



hich 



direction, there being, as 



may eafily feparate in an hoi 



coheiion between 



e 



T 

ft 



ha 



faid before, little or 



urn and another 



The 



part of the earth that is firft raifed, being bent fi 



form 



W] 



ill 



a 



d the 



md 
par 



our to reftore itfelf by 
next to it beginning to 



ght fupported by the vapour, which 



its elafticity, 

have their w 

v/ill infinuate itfelf under them, will be raifed in their 
tiirn, till it either finds fome vent, or is again con- 
denfed by the cold into water, and by that means 
prevented from proceeding any farther. 



59 

be 



Tf 



•ge quantity of vapour Ihould 



generated for fome time, feveral waves might 
be produced by it^ and this would be, in fome mea- 

fare, the cafe, if the quantity at firfl generated was 



exceedingly g 



though the whole of it was g 



rated in lefs time, than whilft the motion was propa 

gated through the diftance between tv^o waves. 

6o. Thefe waves muft rife the higher, the neare 
they are to the place from whence they have 



great diftances from thence, they 



fource 



but 



may rife fo little, and fo 




as 



not to be per 



4 



ceived, but by the motions of waters, hanging branc 

in churches, &c. 

6i. The vibratory motion occailoned by the firfl: 
impulfe of the vapour, will be propagated through 



the folid 



par 



of the earth, and therefor 



It wi 



much fooner become too weak to be perceived, than 
the wave-like motion ; 



for this latter, being 



fioned by the vapowr infiauating itfelf between th 



/ 




ata 



7 



/ 







-^T1 



^ 



■-H 








r 



39 

ftrata, may be propagated to very great dillances; 
and even after it has ceafed to be perceived by the 
the fenfes, it may Hill difcover itfelf by the appear- 
ances before- mentioned. 



S 



IV 



6 



All 



earthquakes derived from the fame fub- 
terraneous fire, muft come to the fame place in the 
fame diredion ; and thofe only which are derived 
from different fires, will come from different points 



all probability 



feldom 



of the compafs -, but a 

happens that earthqu 

affed: the fame place, 

that they come from the fame qii 

how^ever, to be fuppofed, that this 

the cafe, for it will; probably, fometimes happen t< 

be other wife : and this is to be expeded in fuel 



fed by different fires, 
efore find in general, 

■ : it is not, 
Id always be 



cii 



places as are fituated in the neighbourhood of feveral 



\ 



•fubterraneous fi 



s 



or where, bein^ fubjed: 



F- 



of feme local earthquake of fmall 



thev 



now and then are affcded by an earthquake, produced 

by fome more difl: 



but much 



fiderable 



c 



aufe 



Of this lafl cafe, we feem to have had fome 



1 (lances in the earthquake of the iff of November 
/^-f, and thofe local ones, before-mentioned, which 

.icceeded it. 
63. As we may reafonably infer from many earth- 
fame place, from the fame point 



comin 




q 

of the compafs, that they 



all derived from 



fame caufe, and that a permanent 



io we may 



fonably infer the fame thing alfo, from their being 



propagated with the fame velocity; but thi 



s 



fiill come with 



t2 



fo 



1 



f 



argu 



be 



conn 




% 



*v# 









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^-- 



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t 



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ml- V^V 



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LH 



liiii:! 



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t hi-f*!' ; ;. 



I ■ I ^ 



M '■ ■ ■ ' • '- 



1' ! ' 




^m\ 



ill 



L 



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life- 



!!■!! 










.ti;'r 



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..^w. 






coniidered, tjiat the velocity of any vapour, which 

infinuates itfelf between the ftrata of the earth, de- 
pends upon the depth of it below the furface ; for 
the deeper it lies, the greater will be its * velocity 



We may therefore conclude, from the famenefs of the 
velocity of th,e earthquakes of the fame place, that 
the caufe of them lies at the fame depthj and from 
the inequality of the velocity of the earthquakes of 
different places, that their caufes lie at different 



depths. 



Both thefe are perfedly coniiflent with the 



fuppofition, that earthquakes owe their origin to fub- 
terraneous fires, iince the flrata in which thefe fubfifb, 
may be ealily conceived to lie at, different depths in 
different parts of the world. 

S E C T. V. 

r 
\ 

■ ■ M 

. 64. From the fame caufe, we may eafily account 

for thofe local earthquakes, which fucceed the greater 

and more extenfive ones. If there are many fubter- 

raneous fires fubfifting in different parts of the world, 
the vapour coming from one fire may very well be 

fuppofed, as it paffes, to diflurb the roof over feme 

other fire, ar^d, by that means, occafion earthquakes 
by the falling in of fome part of it : and this may be 
the cafe, in fome meafure, even where the vapour 
paffes at fome fmall diftance over the fire ; but it will 
be mofl likely to take place, where the vapour eithe 



r 



The 



paribus 



am not miftaken) in the ratio of the depth below the furface. This 
feems to follow from a known law of all elaftic bodies, according 
to which they tend to return to their ftate of rell:, when either di- 
lated or comprefled, with forces proportionable to the quantity by 

5yhich they differ froni their natural bounds, 

pafles 



* 






_^fl«ME^ 




-^^ 




-lUl; 



t 















41 

at fome diftance under it, Or betw 



ien tb 

jftratum, in which the fire lies, and that next abov 
or below it. 



le 



,V 





R 







^ 




a 



f 




N 



Sect. I. 
the former part of this trad, I fuppofed 

part of the roof over fome fubterraneous fir 
n : this is an event that cannot happen merel 
dentally 5 for fo long as the roof refls on the mat 



fall 



fi 



part of it can fall in, unlefs 



now 



below could rife and take its place : 

difficult to conceive how this fhould happen, unlefs 



y 



was 



fiffures of the 



ife by fome larger pafTages than the ord 



th, which feem much too narrow 



for that purpofe ; for, befides that the melted matter 
cannot be fuppofed to have any very great degree of 

fluidity, it muft neceffarily have a hard crufl formed 



V 



upon it, at all the fillures, by the long continued con- 
tact of the water contained in them : thefe impedi- 
ments feem too great to be overcome by the difference 
of the fpecific gravities of the part that is to fall in, 
and the melted matter, which is the only caufe that 
can tend to make it defcend^ the manner therefore, 
in which, I fuppofe, this event may be brought 
about, is as follows : 

66. The matter of which any fubterraneous fire is 



compofedj mufl be greatly * extended bey 



ginal 



F 

* As all bodies we are acquainted with are liable to be ex- 



ten 



ded by heat, there can be no^oubt of its being fo in this cafe 



F 



likewife 





\ 



I- ^ > 



k 

c 

ii 



ii 



r 



1' 



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t 



■ d 



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, JT^i 



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r 





If 



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h^i. 1 



\ii^w-\: 



...Mil!! 



iVltSS'!>i''t 



L 4 



•♦if 



■I J 



lf^ 






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.:"!;;! 






:■ ;;i!f;'t 

I V- nr r 1. { I 



'1*1'. : 



' . 



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i'f^ 



I ■ 

■■'■\ 






„ ■■■" 

Li 






i'^^r; 








11 




/■ 



\ 



-* 






?mal dimerifions by the he 
about 



As this will be bi\.^^ 



/" 




row 



adualiy, whilfl the matter fpreads itfelf, or 
hotter, the parts over the fire will be gradually 




raifed and bent : and th 



bending 



will, for fome 



time, 



^.^ 



without any other confeq 



b 



3 



fire continues to increafc 



th 



earth will 



at 



as 



lail 



begin to be raifed fomewhat beyond the limits of 



By this me 

ed_ 

fedion of which 

r 

fire, will be two 



ular fpace will be formed at the 




the fire, and furround 




It, 



tical 



ace, through 
Ions; triandes, 




a diameter of the 
the fiiorteft fide or 



bafe of each 
fides 





xt the fire, and the two long 



being formed by 



which will be feparated f 
proportionably 



upper 
r a c 



d lov 



VQV 



fir 



fiderable extent. 



th 



diftance through which 



are 



fed from each other *. 



This 




/ 



they 
s will be 
gradually 



s 



llkewife ; but the matter of fubterraneous fires is yet much more 
extended, than thofe bodies which are only capable of being melted 

into a folid glafs, if we may judge of it from what we fee of vol- 

for the lavas, fciari, and pumice ftones, thrown out from 



canos 



thence, even after they are cold, are commonly of much lefs fpe- 
cific gravity, on account of their porous fpongy texture, than the 
generality of earth, flones, isfc. and they frequently are even 
So-hter than water, which is itfelf lighter than any known follil 



fuppofed 



bodies, that compofe ftrata in their natural ftate. 

* In Fio-. 4. A is fuppofed to reprefent a vertical fedion of the 
matter on'^fire; B B, parts of the fanie ftratum yet unkindled j 
CC, the two feaions of the annular fpace, (furrounding the fire) 

" to be filled with water, as far as the Itrata are 
^, .-e feveral fets of earth, ftones, &c: lying over 
tbeTre7 which are raifed a little, and bent, by the expanfion of 
the matter at A. As it is not eafy to reprefent the things above 
defcribed in their due proportions, it may not be araifs, m order_to 
prevent the figure here given from miileading the reader, to give 

fooie random meafures of the feveral parts, fuch as may probably 

approach 



feparated ; D 



:-ni|Pr 



If;;.;: ■ 

Mi 






\ 



r 



\\ 



cr* r\ 



-1- ^*- -. 



f 



rJ 






^^ ^. 



k'l^ 






1 




43 

F 

gradually" filled with water 
melted matter 




as It is formed 



the 



being prevented from fillin 




it, by 



its want of fluidity, as well as on account of the 
other circumilances, under which it is to fpread 
itfelf : for the lentor and flugglfl^nefs of th 



kind 



of matter is fuch, th 



hen fomewhat cooled on 



furface by the contad of the air only 



will 



flow, perhaps, ten feet in a month, though in a very 
larcre body : inftances of which we have in the lavas 



of ^tna, Vefuvius, &c. It 
then, that it fhould fpread far, 
tad with water at its ed 

I -. 

and when it is. 




e*^ 



i not to be expeded 
vhen it comes in con- 
foon as it is formed. 



perhaps, feveral months in acquir 



icknefs of a few inches : but it muft, by d 






form a kind of wall between the fire and the open 



g 




before defcribed 



Th 



is 



wall will gradually increafe in height 



becomes 



too tall in proportion to its thicknefs, to bear any 



long 



jche prefli 



of the melted matter : which 




r 



^ 



n. 



I 



M! 






i^!'' 



:7-\f 



i >■■■ 

f 

I 1 
4 , 

r 



» 



w- 






i. 



r> 



r ■- 

approach towards thofe which are fometimes found in nature : we 
may fuppofe then the ftratum B to be, perhaps, from ten or twenty 
to a hundred yards in thicknefs j the greateft height of the annular 
fpace C, next the fire, to be from four or five to ten or fifteen 
feet, and its greateft extent, horizontally, from ten or twenty to 
fifty or fixty feet ; the horizontal extent of the fire at A, may be 
from half a mile to ten or twenty miles ; [See art. 29. and the note 
to art. 53.] and the thicknefs of the fuperincumbent matter at D, 
may be from a quarter or half a mile to two or three miles ; the 
number of the laminas alfo, into which it is divided, may be many 
times more than thofe in the fip;ure. As to the perpendicular 



\ 



fiffures, they muft be fo numerous, and fo fmall, in proportion 
to the other parts, that I chofe rather to leave them, to be fupplied 
by the imagination of the reader, than attempt to exprefs them in a 
manner^ that could give no adequate idea of them at all. 



F 2 



mufl 



I, 



/ 



1 ^ F 

'ft,' 







;i' i 



!M^ 





f 



%"■ 




1-1 -T 



\ 



■- 



\ 





\ M^' L' 

I I 4 I 1 



r . 



i I 



Mil ■' ■ ^'^ 



m 
III 






I'i-ui:!!';!!'! 



'(bm^'jKji 



•f.i 



II 



If"! ' 

" .1 .'(ir; ! 



■ ,; ^\: 



^( -^^^i .1!! .: , 









^^^''■' 



r' ;''. 






r r 










^■,%m 



LI 




i^iil^M I 






: 1 



i| .1 



■i^[| 






r^ :nri I 



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^ ,1 K 






^7- 



i! ;;(.ir,! 







';iv.. 



'i- 



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1 ',5Bi|^^;|l! 

; I. V' ! 

'■ I 

Li ■ 'I ' 



h h 






m.:i:\ 



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;il:^ i 




44 




muft neceffarily happen 

nefs of it will not exceed 



laft, becaiife the thick 



67. Befides the giving way of this wall, the fire 
may undermine the fpace containing the water, and, 
by that means, open a communication between them. 



fuppofe 



one 



thefe come to p 



L 

time arrived when the partition be 



d 



yield 



If 



then the water had any vyay to efcape readily, the 
breach would be made, and the melted matter would 

s quan- 

but as this is not the cafe 



burfl forth immediately, and flow 




titles at once amongfl it ; 
and it can only efcape by oozing flowly between the- 
ftrata, and through the fiiTures, the way that it came, 
the breach will be made gradually, from whence we 
may account for fome appearances that have preceded 

great earthquakes. 

68. We are told, that two or three days before 

an -f earthquake in New England, the waters of fome 

wells were rendered muddy, and flank intolerably 



d 



* This limit will depend upon the thicknefs cf 



r 



fary to prevent fo quick a communication of the heat pr cold 
through itv as that the water fhould be able to diminifh the heat 
of the fire confiderably,.. The thicknefs requifite to do this, is very 
different in different kinds of bodies. Metals of all kinds tranfmit 
heat and cold extremely readily y_ but bricks and vitrified fubftances- 
(with which laftvve may clafs the matter under our prefent confi- 
deration) tranfmit them very flowly : the walls of the hotteil of our 
furnaces, when built of bricks, and eighteen inches thick, will not 



tranfmit more heat than a living animal can bear without injury^, 



thougb the fires are continued in them for ever fo long a time;, 
probably, therefore, if we allow two feet for the thicknefs of the 
matter, cooled and rendered hard by the contad of the water, we 



fhali not underdo it. 



p. 68g, 



Tranf. N" 437. or M 



km. vol. viii 






V 



•■- ".!• 



^ 




Lh II 



I L 



•li 



-% 



i\ 



■ 



r^ 



impre 





why might not this be occafioned by the water 



tained in the fpaces before defcribed, which, being 



fulph 



fleams, were driven 



up, and mixed with the waters of the fpring 
leaft, there can be no doubt, by whatfoever means 
was brought about, that this phsenomenon was owi 
to the fame caufe, already beginnino; to exert 



which afterwards 




rife 



felf, 

the fucceeding earth- 



quake. 



6g 



w 

Something hke this happened before the 




reat Lifbon * 



told, 
then 



that at Col 



h quake of 



7SS 



about twenty 



cc 



We are 
miles from 
I I ft of No- 



(S 



cc 



<c 



<c 



(C 



cc 



cc 



cc 



cc 



cc 



cc 



tt 



cc 



(C 



mber 

eafed 



the afternoon preceding 

e water of a fountain was greatly de 

1 the morning of the i ft of Novembe 



itr 



very mudd 



d after the earthquake, it re 



nels. 



ned to its ufual ftate, both in quantity and 

The fame author fays, a litde lower 



cc 



m 



the afternoon of the 24th, I was much appr 
iive, that the following days we ftiould h 



other 




:h quake 



for I obferved the fame 



pro 
tha 




noftics as in the afternoon of the 31ft Odob 
is/' &c. 



" And I farther obferved 



the 



water of a fountain began to. be difturbed to fuch 



degree, that 





ht 



of a yellow clay 



colour; and from midnight to the mot 




of th 



cth, I felt five fhock 



of which feemed 



*« me as violent as that of the 1 1 th of December." 

70. But the moft extraordinary appearance of any 
that preceded this earthquake, was that of the a 




-4 



^■ 




\ 



i 



i A 



H 



-. 




' ■ L 



i .' 








M 






t 



i 



J h 



!i^ 



I 



'^> 



m 



'* , 



V. 



i!^ 



\ 



* 



See Philof. Tranf. vol. xiix, p. 416 and 417,-01 Hift. and 



Pfeilof. of Earthq. p. 313 



tioa 



\ 



t . 






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-**-#ii 



1 



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t 



( :ji ill" " 



L I 




1h 1^ t' ■ 1 1 



I 11 I ^'11 



u 

'if I 



'■It 






'!', 









t. ti 



it 



■-'i '. 



i - 



iiiilsiffl'i 



!.■( 



•1 1; 



Si! 



n hi 









iltiimillllii 






i 



■ 1il::^•',i^' 
H'i'*'|'■>l 



^^^i 



,! H''l 




I '''iSniii'sik 




■r'^'' ■ ':\\ 



■ ii; 






ii'".' 



\ I- 



[1 









I'Sl.',!!"! i 

■ it'll: ■■ ,. 
l!)«.Vi||i. 









n-\ 



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■ kii^-' 









t 
^ 



• 



t 

I 









46 

-tion of the waters of * Lochnefs, and fome others of 
the lochs in Scotland, about half an hour before any 
motion was felt at Lifbon, notwithflanding the caufe 
of all thefe great effeds could not lie far from thence, 
and, I think, certainly lay to the fouth of Oporto. 
Nor is it probable, that there fhould be any miftake 
in the time, not only becaufe the difference is too 
great, as v/eil as the concurrent teilimonies too many, 
to admit of fuch a folution; but becaufe they men- 
tion another greater agitation, that happened about 



an hour and half after the former ^ which latter agrees 
with the times, when the agitations of the waters 
were obferved in England, if we allow only a proper 
interval for the motion to be propagated fo far north- 
Ward, proportionably to the time it took up in tra- 
velling from its original fource near Lifbon. 

71. Thefe appearances feem to be connecfled witk 

that mentioned in the preceding article, and they 

may both, I think, be accounted for, by fuppofing 

a confiderable quantity of vapour to be raifed, 

whiifl: the partition before-mentioned was begin- 

which time, a partial 



nin 




to give way 



during 



r 

* See Philof. Tranf. voh xllx. — or Hift, and Phllof. of Earthq* 
art. Lochnefs, Lochlommond, &"€. .The fame thing alfo feems 
to have taken place in Sv/itzerland ; for Monf. Bertrand fays, that 
all the agitations of the waters in the lakes there, which were ob- 
ferved on the ifl November 1755, happened between nine and ten 
in the morning; and particularly at lake Leman, he fays, the agi- 
tation happened jufl: before ten ; which, allowing for the difference 
of longitude, muft have been juft before nine at Lifbon ; and, con- 
fequently, if there is no miPcake in the times, all thefe agitations 
preceded the earthquake, at this laft place, by near three quarters 
of an hour. [See Memdres fur les trembkmms de Terre^ p. 107 



J 



communi 



I 



I 



"^ 

J 



. \ 



r 



— - -1 






«^ A, 






I 



.^ 



/ 




47 




V 



communication between the water and fire would b 



u 



broug 



d that by de 



only 



H 



but 



vapour,, noi being produced at once 
might creep * filently between the itrata 




ally, 
'ards 



that quarter where the fuperincumbent mafs of earth 



was 



li 




tteft; and 




means, fome places very 



fource of the vapour mi 




ht be 



at all, 



fFecled by 



affeded, thou 




whilft others mi 
ley lay at a g 



ht be 
diftance 





thofe places, which lay immediately 

part where the vapour was paffin 




ny efFed 



of the 




motion 



ccafioned by the fmall quantity of 



or not 
reatly 
\ and 
r the 

might not per- 

fs of the 
it. This 



^ 



cafe 



might continue to be th . 

country where- the fet of ftrata above beiri 



came to fom 




much 



thinner, the vapour would not only be hurried fo 



r 



ward, but colleded alf 



u 



pafs; 

prodn 



and therefo 



mor 



ch narrower com- 
raifing the earth more, would 

and this we ought 



^ 



fenfible effeds 




* Some appearances that have been obicrved in New Eng.and 
feem to confirm this, and make It probable, tnat a fmall quantity 
oftapourl often found to creepfilently between the ftrata, before 
aUneral communication between the water and the fire g.vesnfe 
toihe ..reater and more fenfible efFe,as of earthquakes. See Philof. 

or Martyn's Abr. vol. viii. p. 693. where we are 



Tranf. N 



told, that, at Newbury, a little beforeany noife or fhock was p^- 
ceived, the bricks of an hearth were obferved to nfe, and, falling 

to lean another way. In the fame account, it ,s aifo 
a few minutes before any ftock came, many people 

• ' ir ftomachs:' an eftedt. 



down asain, to lean another way. 
faid, that " 



«^ could foretell it by an alteration in their liomachs : an ettect, 
wlSh feems to be of the f^me kind with fea-fickaeft^, and waich 
S:;ays ac:;Lpades the wave-like moti^. of earthquakes, when .t 



is fo weak, as to be uncertainly diftinguubab.e. 



J' 



cliieily 




\ 



ki I 

H 



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1 ■ . 

L \ ■ 



^■' 



4 ^1 



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if 




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:ii|if 






f 




- I 



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ij|t...5fui|; 







'iIN;::, 



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•ir'-';: 






f^ * 



LI r^i 






hi I I' 



1 r 



' ^l ■ L h ! I 






I J 
F 

I I 






ii'i^' 



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■I 

.. ''I; 



■r'%i/;:i.f|||.l,i 




r \ 









liiu- 



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f ■ 






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■ ■lit 



I '■'ifiiij-:. ■ .-'■ 



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n 




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. I 




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; 'iifi r . 






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.(i 



r.'.'V.iiftii! 



3U 



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48 



/- 




P 







7 

fe 



Hy to exped in the moft mountainous 

idea before given of them 



ord 




To 

in Fi 



ake this fomethin 



ft 




I. 



th 



t> 



d, 



in the dotted 



vapour to be paffing b 



fup 



C 



d 



d 



A : whilft, the 




ap 



t^ 



P 



/^ «* 



r 



little, 



parts at E, it 
IS well becaufe 



aife the 



forward 
paffes under tl" 
arth over it bi 



be fpread broader and 



wei 



rives 



becaufe it will be more comprefled by 




of the fuperincumbent 



b 



as It ar 



ds A, not 




the latter 



P 



w 



driven forwards with greater velocity, b 
moft will travel flower, on account of it! 

ita : and, beiid 



be 



fore 



under a 4- thinner fet of ftr 




d being much lefs 



It wi 



From all thefe caufes taken 




tly expand itfelf. 



furface of the earth, occafioned by the paffing of 



vapo 



nder 



will not only be much h 

and, confequently, the ; 

jnts, will be much mor 
d, mor 



b 



aifo much fliortei 

.on both thefe ace 
to the horizon: i 

of th€ wave will be flower, it wdll sive moi 



^ 

u 



becaufe the p 




led 
efs 



any waters fituated on 



fide of 



flow 



one 



way J and on this account alfo, the apparent ag 
of them will be increafed. 



Sec t, II. 



73. We are told, that, in the Lifbon earthquake 
of 175-5-, "the bar [at the mouth of the Tagus] was 
" feen dry from fhore to fliore 3 then fuddenly the fea, 



* See art. 43, 

+ See art. 63. the note. 



<c 



like 






V 



^ 



A 



i ^^^ :^ 




J ^ I 







49 




ec 



cc 



<c 



cc 



•cc 



cc 



like 



came 




in 



and about B 



lem caflle, the water rofe fifty feet almoft in aii 



ftant 



and, had it not been for the 




bay 



ppofite to the city, which received and fpread 

le great flux, the low part of it mufl have b 



under 



The fame phenomena were ob 
ferved to accompany the fame earthquake at the ifland 



of Madeir 



where we are told, that, at tl 



le 



city of 



Funchal, " the fea, whicn was quite calm, w 



/ 



cc 



cc 



cc 



cc 



cc 



to retire fuddenly fome paces; then rmng 



ferved 

with a great fwell 

fuddenly advancing 



leaft noife, and as 



flowed the fhor 



d 



ed the city. It rofe full fifteen feet perpen- 
dicular above high-water mark, although the tide 

et, was ther 

f the ifland 



^ I 



" which ebbs and fiow« there feven fe 
«* at half ebb. In the northern part o 



V 



cc 



cc 



cc 



cc 



cc 



the inundation was more violent, the fea retiring 
there above one 



hundred paces at firft, and fud 



denly returnln 




verflowed the fli 



for.„.^ 



open doors, breaking d 



the 



alls of fe 



ma 




^ 



d florehoufes, and carrying away 

** its "recefs, a conflderable quantity of grain, 
fome hundred pipes of wine f 

ppearances 



d 



74 



B 



thefe 



which hav 



I 



obferved to 



d fe 



other 



thquakes 



as 



11 as this) feem to admit of an eafy fo 



fup 



poflng the caufe of them to lie under the bed of 



ocean 



the farther progrefs of the communi 



for. 
between the fire and water, the vapour, th 



IS 



* See Hift. and Phllof. of Eartbq. p. 316. 

t See Philof. Tranf. vol. xlix. p. 432, ^^V.— or Kid. and Philof. 



.©f Earthq. p. 329 



G 



^ 



gradually 



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I k 





m 



i'»' 



cfvi' 



I , 



t 



'^«^: J^ 



~rU'' >.,'rt 



, \ 



•I 







"ill' l''*,»l'' 




] ■ 

\ 

I 

Ik 



,1 ^** 



I r 



L L ■ 

h 

■ ■ :;tl 









gradually raifed at firft, will at laft begin to raife tlie 

oof over the fire, v/hich, being fupported by fo light 

vapour, there will now be no want of fluidity in the 



matter it 



fts 



upon 



d the diffe 



gravity between the two 



of fpecific 



of being fmall, will 



be very g 




hence, if any part of the roof gives 



way, 
rifin 



muft immediately fa 




d 



4- 



aking its 



once made, a 




the vapour readily 






nd a beg 




b 




com 



berlefs clefts and fifli 



ation v^ili be opened with 



falling in of vail quantities of 



ft OGcafion the 
^/hich, as foon 



the vapo 



pafs round; them, will 



t 



the 





fupportj then will follow ...^ ^ 
defcribed. 

/ 

y^. Now, whilft the roof is railing, the waters of 



the _ 

thence 



lying 

y way 



muft 
this, howe 



and flow from 
being brought 



J 



about flowly, they will have time to retreat fo gently 
^s to occaflon no great difturbance : but as foon as fome 

part of the roof falls in, the cold water contained 



the flflures of it, mixing with the fteam, wi 



V- 



J^ 




diately produce a vacuumy in th 



fame 



manner as- 



/ 



water injedled into the cylinder of a fteam eng 



d the earth fubfidins^, and 



g 



a h 



w pi 



r- 



above, the waters will flow every way towards it, 
and caufe a retreat of the fea on all the fhores round 



> 



abou t 



then prefently, the waters being 




am con 



erted by the contad of the fire into vapour, togeth 



h all the additional quantity, which has now 
open communication with it, the earth will be raifed 



and the waters over it will be made 



^— 



flow every 



/ 



'^ See art, 56 to 60 inclufive 



l-r 



/ 



wavj 



i 




and produce a great wave i 
ing the previous retreat *. 




Sec t. 

quantity of water, which we h 

upon.fubt 



fuppofed to be let 

by that means, to produce earthq 

us with a reafon, why they obferve a fort of p 

cal return. This water mufl extinguifli 
tion of the burnin 




por 



^ 




matter 



d though th 



onfequence of which, 
a(5ted within much narrower bounds ; 

could not 



/ 



effeds before defcribed 



heated 



place at firfl, but by the great exten-fion of 



yet, after they have once taken pi 



ii 



S; 







'-, 



"I . 



1 

H. [iJ 



m 



f 



y 



* It may, perhaps, be objeifled, that thefe pKxnomefia may as 
eafily be occafioned by a vapour generated under the dryland", 
which, by firft raifmg the earth upon the fea-fhore, would make 

the watervS retreat i and that the return of them again, upon its 
fubfiding into its place, rnight caufe the fubfequent wave. That 

this may be the cafe, in fome inftances, is not impoffible, but, I 
believe, upon examinijig the particular circumftances, it will ge- 
nerally be found to be otherwife ; and there cannot be any doubt 
about it, in the cafe of the Lifbon earthquake^ for the retreat w^as 
obferved to precede the wave, not only on the coaft of Portugal, 



Mad 



now 



if the retreat had been caufed by the railing of the earth on the 
coaft of Portugal, the motion of the v/aters occafioned by this 



means 



Mad 



there previous to the retreat, contrary to what happened ; nor 
ould the motion of the wsters at Madeira be caufed by the earth* 
<]uake at that place, becaufe it did not happen till above two hours 



been 



continuation of a motion propagated from the place, where the 
earthquake exerted its firft efforts, ^ind we may obferve, in gene- 
ral, that this muft always be the cafe, whenever the retreat does 

not happen till fame confiderable time after the earthquake. 



G 



2 



they 



/ 



I 



» ■- 



- 1-' 






h 






V 



1 1*. 

1.1 



"» 




■<' 



I 






'^ 'r' 



:-f 




■'i 




'i^^-^'M 



- \ 



IS 



1 .■^L: 



'■ iiiT, 

■ ^ftiiiiiii"'"'' 



'^VAP^ 



'.]'.■ 












iiii 



l'^ 



^^ii 



M. I 






■^^ii 



T ■ il'. 



lift. IV- "i-*^ 






rh 











vj. 






' ,! 'X\ 



'f'V.',..M[ 






^ 



!:i|i. 









■' Y 



If- I L 

■ I '. . "' 



■ -J 



HT*^"^ -til 



r 



difturban 



falling in of 

frequent communication 




do fo for fome time 

inftance, 
part of the roof, muft render the 




fire and 



ly very eafy, but almoft unavoidable 



d this 



will continue to 



roof 



f( 



and the furTace of the melted matter 




cooled, after which 



may require 




for 



the fire to heat it again fo much, as will be necefiTary 



make it produce the former effed 



Now 



th 



e 



matter has been m 



cooled 



the com 



buflible materials are with more or lefs difficulty fet 



fi 



t) 



of other circum 



/ 



ftances, the returns of thefe effeds will be 
earlier i but though they will not, for this reafo 



obferve any exad period, yet they will generally fall 



within fome fort of lim 



either the matter th 



occafions them is confumed, (which, probably, will 

feldom happen in lefs than many ages) or till the 
fires open themfelves a palTage, and become vol- 



canos. 



r 

Sect. IV. 



E 

77, I have already intimated, that me moil exten- 
five earthquakes frequently take their rife from the 



lea 



Accord 




the defcription of the * ftrud 



of the earth befor 



fi: 



g 



y combuftible fi;ratum 



at 




depths in places under 



/ 



than elfewherej, hence far more extenfive fires may 
fubfifi: there, than where the quantity of matter over 
them is lefs^ for any vapour raifed from fuch fires, 



* See art. 43. 



havin 




\ 



'Am 



■ ht ' 



. ^■'.■rt»l 



1'^ 









\i 






m 




It 





53 

having; both a flronger roof over it_, and bein 




by 




wei 




felf, 



b 



e 



preiled 

hC (befide the additional Vv-eight of 

w^atTrTwiU not ordy be lefs at liberty to expand 

and confequently of lefs bulk, but it will alfo 

filv driven away towards the parts round about, 



here the fuperincumbent matter is lefs. and th 



rp' 



1 112 



fo 
from 
light 
fo ei 



liter. 

fires, 



& 



On 
whe 



other hand, any vapour raifed 



th 



fuperincumb 



matter is 



findin 



eafily d 
cd heavie 

pen a m. 




weaker roof over it, and bein^ 



ii 



away und 



ft 



that are 



thick 



will be very apt 



break 



o 



er 



d 



ons; before the fi 



id it muft neceffarily do 
have fpread them 



/ 



fufficiently, to be near equal 
fubfift in places that lie deep 



to 



ofe which in 
All this feems 



y 



be greatly 
which are 



firmed by th 



fit 



f 



ft 



ys 



mountains, and thofe of 

the world 



found on 
fome of 



the 



* 



tops 
g;heft 



of 



7 



8. If, then, the- largeft fi 

fubfift under the ocean, " 



It is 



be fuppofed 
der that the 



-»^ 



r^ 



* Perhaps this may fupply us with a hint (if the cdnjeaure is 

not thought extravagant) concerning, the manner m whicn thefe 

mountains have been raifed, and why the ftrata he generally mo.e 

iaclinino- from the mountainous countries, than thofe^countnes 

themfelves; an appearance not eafily to be accounted for,. but 

^ the ^ppofition, that the upper parts of *--«^,-^^P- 

matter, in fome degree, though not perfedly fluid and that .ni. 

ma te is hghter than^the earth.that reSs upon it. Th. conjeaure, 

however, will probably be thought lefs firangc, if it be conhdered, 

•thauhe new iLds, formed abSut Santerini-and the Azores have 

fome of them been raifed from 2C0 to 300 yards, and upwards ; a 

height which might well enough intitle them to the denommaticn 

of mountains, if they had been raifed from lands not lymg under 

the ocean. [See Fig. 3.] -_ 



moft 



^ 



i 



J ' ^ 



• >*■ . 



•^^ 







i L 



4 



^], 



I .- 



111-.; 



■'.?i 







HI 
I? 










k 



a 



1 



L 



i' 






1 

> 



It-; 



\ 
t 



t 




-I 



I 



Hf 



1 ■-_ ■ r^ 





■ =■ I 



I 

" I 



H h 




'1^ 
1^ 



I, h 




i\ 



Hi 



"i 






r 'T ■'. nil 



I ► 



:;;i 



p. 







!l ^^"■'"'Wll. 



I >■ I 




Ik 
I . I 1 



I? 



■ f . ^' 






n 



:':.J^^'t 




\^m:p 











I ' I 







u 



\ 





thence 



th 



54 

five earthquakes (liould take their rife I-otn 



e 




reat 



iliewn to have done fo 



hq 



ike of Lifbon has 
d that the caufe of 



been 



alio at a greater depth, than that of many others, ap 



prop 
7< 



from 



<yated 



•f- velocity with wl 



t> 



ine 




reat 



d Callao in 1746, d 



thquake that deftroyed Lima 



to 



h 



/^ 



from 



f 



a 



for feveral of the ports upon the coaft wer 



^ f 



overwhehned by a great wave, which did 

four or five minutes after the earthquake bepan, 

which v/as preceded by a t retreat of the water 



and 



as 



Lifbon 



A 




be alleged, that there were fo 



fuddenly, in the neighbour 



nfl: this, it may, per hap 




mount 



thquake happened, and th 



the B 



broke out 

when this 

of thefe 



^... be the 



*^ 



cafion of 



This however, I think 



IS 



y probable; for, to 



om 



wave, and previous retreat of the wat 

y likely, that mor 



the argument of 



mentioned 

fire was concerned : befi'des 
felf a 



,1 



pofed 



pafiage at thefe pi 






already 
an one 

pour, opening it^ 

Id not wxll be fup 



far ; 



it took its rife from thence, to fpread itfelf 



efpecially towards the fea, where it is manifefl 



N 



) 



4 , 



till 






"1-1 



'■Mm- 



v 



if' 













i^. 



\ P 






?*!!!; 






^^^|i; '^ 



'ml 

I "i™ ' ■ h, ! L 



* See art. 54 



t See the note to art. 63. 

;|; Both the wave am 
er great earthquakes 

1,. \ 



L 

See alfc) art. 94 to 97 inclufive. 



Other great earthquakes^ which have happened at Lima, and m the 
nnghbouring country. See d'Ulloa's Voyage to Peru, part ii. 
book i. chap. 7, 

If thefe volcanos v/ere not new ones, but only old ones which 

^^ _^ _ J ■■■■■■ ■ ^i^^ ^ 



with fiill greater force. 



[See the note to art. 34. 1 



/ 



r 

that 






ii?if>i 




V 



"t 



\ 




\ 



tliat the flrata over it were of great thlcknefs, as a 

pears from the great velocity with which the earth 



tinued with equ 



propagated 



I 

ly equal 



months after the openings were made 5 

fires had been the caufe of them 



for fome 
hereas. 



ceafed, 
happened 
more probabi 



other 



immediately 
a vent, 
therefore m 

quantity of vapour, takii 

more exteniive fire under the fea, fpread itfelf fi 




fires finding 

cafes. 

y large 

me 



then 



as It 



!-/=» 



over it was naturally much thinner, as well as greatly 
weakened by the undermining of thefe fires, it opened 
itfelf a pafiage. and buril forth 



lo. As the moil extenfive earthquakes 





snerally 
proceed from the loweft countries, but efpecially 



fi 



,e 



fea, fo thofe of a fmaller extent are g 



lly found 



i 




ft 



hence it almof 



always happens, that earthquakes, which are felt 



fea, if 



felt alfo in 



lands 



where 



th 



gh 



and thofe 



y 



are many amongft the hills, 

extend 



violent ones, which never 



tiemi 



the lower countries. 



T 



we are 



told, th 



a 



t< 



■** 



the 



Jamaica, " -f fi:takes often happ 



try, not fe 



Port-Royal 



d fome 



times are felt by thofe that live in and at the foot 



* See art. 28. 



Th 



J 



that hap- 



before- mentioned, was attended with the Vv'ave and previous re- 

iiiL N"" 209. or Lowthorp's Abr, vol. ii. 



treat. 



of. Tr 



7 



£C 



of 



i 

H 



/ 









N 



' h 






.* 



Tl 



P\ 



*■ 



i-' 



jiii 



f; 






xu 



h 



I 

i 



.11 



Si 



,r 



1' 



■w.. 






% 



V, 



I'! 



■■*: 



/' 



^. 



■- 



f- 










W 






. IT 




t . 



„.:j 



- ' ■■; 



\m-A. 













■(■..iifi*^' 






r ' 



H l> 



t' 







k> 



■:".iii 



ii 






ir 






I ■ 



. I 






mm 



^1 ■. 






il%»vf« 



» h 







, ■ ■ .■ 1 



.■ti 




^^(1 



llirfTilkiii 



i;f 



;J 




> 



r. 



■i! 



!i:^ 






,'-i: 



I r± 






* I 



; > 




-,Li-i 



111". 'Wi 

Vf.' 






/ 



L L' 



ih 






i¥ 







\ 



o 



R 



ther 



mountains, and by no body eife." On the 

thquake - that defcroyed Port- 



d 



the 

ded itfelf 



over the illand 



d the 



\ 



/ 



fame was obferved of a fmaller earthquake, that hap- 

ch latter undoubtedly 
by Sir * Hans Sloane's 



P 



ned there in 1687-83 wh 
ame from the fe 



of 



app 



p. in f* 

Lai 



Si. Earthquakes of fmall extent are alfo very 
mon amongfl the mountains of Peru and ChiH. 



An 



CQ 



d'Ulloa fays, ^^ Whilft we were preparing for 



cc 



<c 



cc 



cc 



cc 



cc 



fe 



fou r 



our departure from the mountain Chichi-Choco 
there was an earthquake which was 
leagues round about : our field tent was tolTed to 
and fro by it, and the earth liad a motion like 
that of waves: this earthnuake, h 






was 



of thefmalleftj that commonly happen in th 

; author tells us, in anoth 



Luace, 



P 

cc 



untry. ' 
that. 



The fam 

during his flay 



H._ 



th 



or 



in the neighbourhood of 



ther 



ity of Q 



f^ 



WTre two 



quakes, violent enough to overturn fome 



(< 



cc 



nd 



the country, which buried feveral perfons 



yy 



V 



Sect. V 



8 



it is generally found, that earthquakes in hilly 



countries, 



much more violent than thofe, wh 



h 



a 



PP 



n 



wh 



d th 



s is obferved to be the 



cafe, 



as 



well when they take their rife from th 



lower countries, as 



amongfl 



tl 



le 



h 



th 



em 



fel 



ves. 



This app 



from the ftrudure of 



being fo eafily to be accounted for, 

already defcribed, I 



earth 



1 

^ See Pliil, Tranf. N° 2093 or Lowthorp's Abr. vol ii. p. 410. 




x 



Ip 

^1 



Ii 



\ 



I k 
y 9 



»,■ 



-I 




^jJ 



-^^ 




n 



*. 



-^- 



^ 




57 




fliall content myfelf with eftabiifhing the certainty of 

a faa, which tends fo greatly to confirm * 



8 



Th 



c 



hquak 



h 



infefted fome of 



the tov/ns in the neishbpurhood of Qi 



have 



^Jy been incomparably more violent than that v^hich 
delkoyed Lilbon, but they feem to have exceeded 
that alfo which deftroyed Lima and Callao. ^ In 
* Lifbon, many of the houfes were left ftanding 



1th 




h few of them wei 



lefs than four or five 



At Lima alfo, it is only faid, that 




"all 

reat and fmall, or at leaft; the 

Callao 



ilories high. 

" the building , ^ 

** greatefl part of them, were deftroyed 



likewife 



_^ ^ ppears from the accounts we have of 

it" had' many houfes left unhurt by the earthquake 



the wave came 



which overwhelmed the whole 



town 

way. 

thofe produced by 



Its 



and threw down every thing that lay in 
All thefe effeds feem to be greatly fhort of 



hquake that happened 

Latacunga, in the' year 1698, when the whole town 

cbnfifting of more than fix hundred houfes 



was 



minutes time, a 



..:ely deftroyed in lefs than th 

part of one only efcapingj notwithftanding that the 
houfes there are never built more than one ftory high. 



\ 



order, if poflible 



avoid thefe dang 



Am 



bato 



ge about the fanie fize as Latacung 



gethcr'with a great part of Riobamba, another town 
in the fame neighbourhood, were alfo entirely de 

ftroyed 




the fame 




and fome others 



were either deftroyed, or received confiderable damage 




'* SeePhilof. Tranf. vol. xlix. p. 403- where it is faid, « of 



«« the dwelling-houfes, 

« that tumbled." 



there might be about one fourth of them 



V 



i- 



n 




/ 



II 



•1 






\f.f 



! 



5 

I 
I 








f^r 



!!> 






■It; 



i'A 



/ 



;lf- 



> 
i 



I 

I . ■ 



'1 H 



' ^ : 



11 



I : 



i! 



' -t 






I; 



JT 



L 



,+^ 



} 

jr 



iil 

It,- 
|!> 








\i 









* 



f r 
•I 






y 



\ 



\ 



/ 



V} 






b -% 



-«l_l*' 




;^ . '^i'-l ^i|^[k 



I ! 



J I ■ 



' . ^ 



111 



i 

;Jll 



r r 





>i' 



\- , 




... Q 

. J ft ri ' Lif. I ' .1 ( 









"r! 






w 



'I'l 



■iilf''«iii''r.-'" 



■)1 



^ u'>' 



\\ 










'.ii 



-ll^-s^i 



/ 






■r 






■)^'' 




I 'i f 




'.'W^m 



■ :L 
I 

r 

■ii\ J:' 



'i. 










■;;;;;,,w.-': 



ii 









F t 










I I 






Ki; 






/ 



f 



^'*^ 



-' 





flenly ,n the neighbouring mouniain of Carouavrafo 

as before-mentioned : and. " ne„- A.^uJ'iuJ'^^", 



€C 






<c 



cc 



CC 



before-mentioned 
opened itfeif in feve 
mains 




Ambato, the earth 



the fouth of that town 
five feet broad, and about 



and there yet 



cleft of fo 



g 



gth 



lying n.th and ibuthMhe,;;;:^!,,^^^^ 



of * Quito was affefted 



ed no damag 



the farhe time, but 



two geographical miles from L 



■-^^^ «. uxiw mnic ume, but re 
though It is no more than forty 



whe 

have exerted^itfelf 




far fr 



the greate^ft violence of the /hock feem. .. 

Thefe towns are fuppofed to 



being 



fland by far the higheft of any in the world oeins 
of &S *' '"^- ''^l^'^"^ the tops.f S 

01 tUe higheft mountains in Europe ; and the emu,,^ 
upon wWch Riobamba ftands, 4ms but f^f 

fift n?o?? *"%'rf ^ ^^ ^'S" '' Snowdon th^ 
•njgnelt mountain in Wales. 

it .T^^ ,'^'';^"^7 "P^" .wl^^ch thefe towns fland, 

another fet of 
mountains, which are much the 



ferves as a bafe, from whence arife 
high 



ds 



d 



higheft in the known world 
tains there are no lefs than fix 



Amongft thefe 



if not mor 




Tho 



* The city of Quito ftands lower 

by about 500 yards perpendicular. . „ou2n it elcaoed thi, \t 
.feas lately, however, been deftroyed by anothfr v olent Lr, Ji'^'V 
tiiat happened on the 28th April 1756'TS T h 'fnl? ' 
reen any other particulars worfh notice. ' ^'^'''°'- 

clZ!!!JL!^1i"g to Antonio d'ailoa*. account: burMo 



a% 



«.umpwiiigita!ti77otoifcs 
meridian.l 



[See his meafure of a deg 



^J 



^ 



within 






^ 



V 



i 




\: ^- 



J- 



f *■■ 







\ 




59 

within an extent of 1 20 miles Ions:, and lefs than 



thirty broad 



loweft of which exceeds the heig 



of Riobamba by above two thirds of a mile, and the 
ighefl: by more than twice that quantity 




Now 



as 



hquakes have been more violent at the foot of 



thefe mountains, th 



in the lower lands, fo they 



have been ftill more violent towards the tops of them 
this is fufficiently manifeil, from the many * rents 
made in them, and the rocks that have been broken 



off from them, upon fuch occafi 



ftill more manifeftly, and beyond all difp 



but it appear 



burfting forth of 



/ 



the very 4- fummit of the 



ite, in the 

■ 

which are almoft alway 



found 



where they 



In thefe inflances, the earth, ftones, &c 



which lay over the fire, are generally fcattered by the 
violence of the vapour, that breaks its way through, 
to the diftance of fome miles round about, 

Bf. The great earthquake of the ift November 
^7Sfy was alfo more violent amongft the mountains. 



€C 



cc 



cc 



#c 



cc 



ct 



<c 



the city of Liibon. We are told, th 



<c 



the 



mountains of Arrabida, Eftrella, Julio, Marvan, 
and Cintra, being fome of the largeft in Portugal, 
were impetuoully fhaken, as it were, from their 



very found 
their fumm 



d moft of them 



pened at 



ts, fplit and rent in a wonderful man 
ner, and huge maffes of them were thrown down 
into the fubjacent vallies t." . - 



I 



t 






-'-^ 



H 



'i:*. 



I ■ ^ 

J?; 



'ii-. 



I. 



\¥. 



V-, 




i\ 



')t 




if = 



I 



^ 



.;■' 



f. 



^ 



t 






* 



k 



y 



- 4 



Per 



•-_ \ 



t The only exceptions that I know of to this rule, are in thole 
cafes, wliere the high eft part having an opening already, fome 
frefli mouth opens itfelf in the fide of the mountain. 



± See Hift 



H 2 



86. The 



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6o 

^ 86. The fame was obferved at Jamaica likewife. 
In the earthquake that deflroyed Port-Royal in 1692, 
we are told, that " more houfes were left ftanding 
" at that town, than in all the illand belides. It was 



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arms fpread out, to prevent being 

the incredible motion of the 




fo violent in other places, that people were violently 
thrown down on the ground, where they lay with 
their legs and 
tumbled about 

earth. It fcarce left a planter's houfe or fugar- 
work ftanding all over the ifland : I think it left 
not a houfe flanding at PalTage fort, and but one 
** in all Liganee, and none in St. lago^/except a few 



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low houfes, built by the wary Spaniards. In Cla^ 
rendon precincft, the earth gaped, and fpouted up, 
with a prodigious force, great quantities of water 



into the air, twelve miles from the fea: and all 

over the ifland, there were abundance of openings 

of the earth, many thoufands. But in the moun- 

^tains, are faid to be the moft violent fhakes of all) 

** and it is a generally received opinion, that the 
it 

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nearer to the mountains, the greater the fhalce^ 
and that the caufe thereof, whatever it is, lies 
there. Indeed they are ftrangely torn and rent, 
efpecially ^the blue, and other highefl mountains; 
which feem to be the greateft iufFerers, and which, 
during the time that the great fhakes continued, 
bellowed out prodigious loud noifes and echo-* 



mes 

87' 



cc 



Not far from Yallowes, a mountain, 




having made feveral moves, overwhelmed a whole 
family, and a great part of a plantation, lying a 
mile off 5 and "^a large high mountain near Port- 

^* morant 



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morant, near a day's journey 
quite fwallowed 



is faid to be 



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In the blu 



from whence came 



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thofe dreadful roarings,, may reafonably be fup 



pofed to be many ftrange alterations of the like 
nature -, but thofe wild defart places being very 

or never vifited by any body, we are yet 




g 



of what happened there; but whei 



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they ufed to afford a fine green profped, now one 
half part of them, at leaft, feem to be wholly de- 
prived of their natural verdure *." 

s E c T. yi. 

89.1 have fuppofed, that fires lying at the greatefl 
depths generally produce the moft extenfive earth- 
quakes, we muft, however, except from this rule 
thofe cafes where the depths are very great : for, as 
the weight of three miles perpendicular of common 

-earth is capable of abfolutely repreffing the vapour of 

inflamed gunpowder, fo we may well fuppofe, that 



T 



* See Philof. Tranf. N** 209. or Lowthorp's Abridg. vol. ii. 
p. 416, i^c. where there is a great deal more to the fame purpofe. 
See alfo Hift. and Philof. of Earthcj. p. 286 and 287. 

From the authorities quoted in this fe<9:ion, it appears, how 



the notion, that either large cities, or 



towns fituated near the fea-coaft, are more fubj eft to violent 
earthquakes than others : it is not, however, much to be won- 
dered at, that fuch a notion fliould have prevailed, after the great 
deftru(aion that happened in fo large and populous axity as Liibon ; 
fmce the demolition of a few ruinous houfes only, in fuch a place, 
would have afFeded the imagihations of men more, and would 
have been more talked of, than the fubverfion of whole mountains 
in fome wild and defart country, where at moft half a dozen un- 



feel the efFe<Sts of it, or perh 



diftance 



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be a quantity of earth fufficlent 



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d keep it within its orig 



fFed: 



manifeft, th 



Now, when 



or, 



may happ 



though 



pro 



quantrty of earth may not be fufficient abfolutely to 
rcprefs t!ie japour, yet it may be fo great, as to fufe 



pand but very 



m this cafe 



quake arifing from it would be but of fmall 



earth 



hke motion would be httl 



e or none; the 

and 



Vibratory motion would be felt every-where ; anc 
the propagation of the motion would be very quick 
rhis laft cu-cumftance being almoft the only one 

by which there earthquakes can be known froi thofe 

which owe their origin to fhallower fires, it muftbe 



diftinguilh them with certainty 



very difficult 

it is almoft impoffible-Jo diftinguifh'ihe mlZ'c, of 
the time of their happening in diiFerent places, when 

the whole, perhaps, is comprehended within the fpace 
of two or three minutes, poffibly, however, feme of 

the earthquakes, which we have had in Engl nd may 
have been of this clafs. S^diia, may 



Sect. VII. 



90. If we would inquire into the place of the 
n of any particular earthquake 
lowing grounds to go upon 



g 



we have the fol- 



Firfi, The different direaions 



i 



feveral diftant pi 

the 



in which it 



thefe direction ^ 
fediion muft be 

liable to great difficult 



if lines be drawn _, 
of their common inter 



arly the place fought : but th 




moie to great difficulties 3 for there muil necelTarilv 
be great uncertainty in obfervations, which cannot 7t 



beft 



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beil:, be made with any great precifion, and which 

rally made by minds too little at eafe to be i 



obfer 



of what pafles 



mor 



themfelves may be fomewhat varied, by the 



the diredion 



qua 



e 



the weight of the fuperincumbent matter 



der which the vapour paifes, as 



caufes 



we 



as 




other 



<)2. Secondly, We may form fome judgment con- 
cerning the place of the origin of a particular earth- 
quake, from the time of its arrival at different pi 

t,„i. *!,:„ _ir_ ?- !• 1 1 . ,.^, . . -T 



but this alfo is liable 
thefe method 



great difficulties. In both 



g 



^eg 



however, we may come 



of exadnefs 



amongft a variety of 
different obfervers. 




But 



to a much 
taking a medium 
they are related by 



93. Thirdly, We may come to the greatefl degree 
of exadnefs in thofe cafes, where earthquakes have 



their fource from under the 

fiance 



for 



thefe 



the proportional diflance of different pL... 

from that fource may be very nearly afcertained, by 

the interval between the earthquake and the fucceed- 
ing wave ; and this is the more to be depended on 
as people are much lefs likely to be miflakcn in de- 
termining the time between tv^ro events, which fol 



low one another at a fmall interval, than in obferving 
the precife time of the happening of fome finp;le 
event. ° 

- P4. Let us now, by way of example, endeavour 
to inquire into the fituation of the caufe, that gave 
nfe to the earthquake of the ifl of November 175-5-, 
the place of which feems to have been under the 



ocean, fomewhere betw 



and Op 



(though probably fomewh 



the latitudes of Lifbon 



' to 
the 



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64 

the former) and at the diflance, perhaps, of ten or 





fifteen leagues from the coaft. 



For, 



95. Firft^ The dirediion, in which the earthquake 
arrived at Liibon, was from the north-weft ; at Ma- 
deira it came from the north-eaft^ and in England 
It came from the fouth-Weft ; all of which perfedly 
agree with the place affumed 

g6. Secondly^ The times in which the earthquake 
arrived at different places, agree perfedly well alfp 



* 



with the fame point. 



And, 



compute 
degree of a great circle from Lifbon, and 



^j. Thirdly, The interval between thefe, and the 
time of the arrival of the fiibfequent wave, concur in 
confirming it. That all this might appear the better, 
I have fubjoined the following table,- alTuming the 
point, from whence I compute, at the diftance of 
about a 

a degree and half from Oporto. In confequence of 

this fuppofition, I have added three minutes to the 
interval betvveen the time when the fhock was felt 

at Lifbon, and at the feveral other places. The firft 

column in the table contains the names of places ; 
the fecond, the diftances from the afTunied point, 
reckoned in half degrees; the third, the time that 
the earthquake took up in travelling to each, ex- 
prefled in minutes 3 and the fourth contains the time 
in which the wave was propagated,, from its fource to 
the refpedtive places, exprefled in minutes likewife. 



£ 



quake, 



re<3:ions 



different places. 



(two or three only excepted ) are taken from the 49th volume of the 



Tranf. and the 



To 



muft 



very numerous, I was not willing to quote at Ien| 



they 



Lifbon 



\ 



i 



A 



'" .. 1; • 




( 



LIfbon * 

Oporto * 

" Ayamonte 



Cadiz - 

Madrid - 

Gibraltar ■ 
Madeira - 

Mountfbay 

Plymouth 
Portfmouth 
Kingfale 
Swanfea - 



The Hague 

Lochnefs ■ 

Antigua - 






M^ 



Halfdeg.j MIn. 



2 

3 
6 

9 
9 

XI 

20 

21 

24 

33 
98 



Barbadoes - - - - - 1 loi 



3 

5 



12 
II 
18 

2i 



ap 



32 
66 



^ 



Min. 

r 

■1 

12 



53 
82 



152 
267 

60 



5 



290 
i3<^ 



5^5 
485 



98. In computing the times in the preceding table 

allowance was made for the difference of longitude 

as it is laid down in the common maps, which an 




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* It appears, by all the accourits, that the interval between the 
earthquake and wave, either at Oporto or Lifbon, was not long: 
I have met with no account yet, however, which tells us how long 
it was at the former, and only one which mentions it at the latter, 
where it is faid to have been nine minutes. [See Memoires fur ies 



315,] -Thefe 



P 



ierved, perhaps, to afcertain the diftance of thofe two places from 
the original fource a little more accurately ; but, as the diftance of 
neither from thence could be very great, a fmaH difference in them 
would hardly fenfibly affed any of the others ; from which, there- 
fore, v^^e may draw the fame general conclufions, a^ if they were 
cxad:/ 



I 



not 



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66 



always greatly to be depended 



\ 



I* 



on. The times 
themfelves alfo are often fo carelelly obferved, as 
well as vaguely related, that they are many of them 



fubjea 



qnfiderable errors : the concurrent tefti 



monies, however, are fo many, that there can be 
dciibt about the main point -, and, that the errors 



might be as fmail 



offible, I have not only 



^eavoured to feled: thole accounts that had the greateft 
appearance of accuracy, but, in all cafes where it was 
to be had, I have always taken a mean amongft them. 
In many of the accounts, the relaters fay only be- 



tween fuch hours 



about fuch 



hour 



this 



kind were the accounts of the times of the agitation 
of the waters at The Hague and Lochnefs, which 
vary the moft from a medium of the reft, the former 

g about k\Qn minutes in defed:, and the latter 

regard to 
, from the 



about twenty minutes in~ excefs ^ with 

the latter, however, I muft -obferve, that 

aqcount itfelf, it is probable the agitation happened 
fooner than eleven o'clock, which is the time men- 



•^ - 



tioned. The accounts alfo of the time of the agita 
tlon of the waters in the northern parts of England 
{QQvn to confirm the fame thing *. 

9p. It is obfervable, in the preceding table, 
the times, which the wave took up in travelling 




4 4 



w 

^ As the fhorteft way that the vapoyr could pafs from near Lifbon 



to Lochnefs was under the ocean. 




it might, on that ac- 



count, be fomewhat retarded ; for the water adding to the weight 
of the fuperincumbent mafs, and not to its elafticity, muft produce 
this efFe<^ in fome degree; it is probable, however, that this could 
make no great difference, as the motion feems to have been very 
little retarded in its paflagc from the original fource to Madeira, to 

which place, I fiippofe, it muft have paffed under deeper feas tha» 



would be found in its road to Scotland^ 



{LOt 



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■jjpf 






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the fame propQFtion With th'e dif^ances of th^ 



fpedive 




from thev fuppofed fource of the 



motion 



th 



howeVen is 



\>\^Qi\ 




ft the 



e ^^/ 



point alfumed, fnice it is manifeft, wherever it was 



k 



that it could not be far from Liibo 



well becaufe 



the wave arrived there fo vefy foon after the earth- 
as becaufe it was fo great, rifing, as we are 
the diftance of three miles from Lifbon, to 

The true reafon of 




the height of fifty or fixty feet 
this difpropdrtion, feem 



be the difference 



the 



depth of the water ; for, in every inft^nce in the abovQ 
table, the time will be found to be proportionably 
fliorter or longer, as the water through which the 
wave pafTed was * deeper or fhallower. Thus the 
motion of the wave to Kingfale or Mountfbay (through 
waters not deeper in general than aoo fathoms) was 
flower than that to Madeira, (where the waters are 
much deeper) in the proportion of about three to 

iivev and it was flower than that to Barbadoes, 
(where its eourfe lay tlirough the deepeft part of the 



Atlantic 



ly in th6 proportion of 



to 



three: fo likewife the motion of it from the Scilly 
iflands to Swanfea in Wales (where the depth gra- 
dually dimimfliesfrorii about fixty or • feventy fa- 
thoms to a very fmall matter) was ilill flower than 
that to Kingfale, in the proportion of lefs thaft 



to three:: the fam€ th 




obfervable.with regard 



iliiiiiiiift 




J I. . t. 



-■ -F^ 



V^B 



V 



We 



in 




deep waters, move with a velocity that would carry them round 
the whole earth in a fmgle day ; but as they get mto " _ 
waters, they are greatly retarded : and -we are told, that in the 
river of Amazons, the fame tide is found running up to the tenth 
or twelfth day, before it is entirely fpent. [See Condaminis Voyage^ 



the Maranon.'X 



^ J 



I 2 



r 

Plymouth 




lfl4 



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J J- 



flh- 



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68 




Plymouth alfo. where the wave arrived about ninety 

minutes later than at Mountfbay, though the difFerence 
ot their diftance from the iirft fource could not, upon 
any fuppofition, be more than forty or fifty miles. 



't. 



Sect. VIIJ. 

100. If we would inquire into the depth, at which 
the caufe lies, that occafions any particular earth- 
quake, I know of no method of determining it, which 
does not require obfervations not yet to be hadj but 
if fuch could be procured, and they were made with 
fufficient accuracy, I think fome kind of guefs might 

be formed concerning it : " 

,./°^* ^^^' ^" *^^^e inftances, where the vapour 
difcharges itfelf at the mouths of volcanos, ( 



for 



cafe of the earthquake at Lima) 



might, perhap 



be poffible for a careful obferver to trace th« * thick- 
nefs of the feveral flrata from thence to the place 

where the earthquake took its rife, or at leafl as far 

took its rife from under the fea^ 



the (hore. if 



If this could be once done 



any one inftance, and 



th& velocity of fuch an earthquake nicely determined 
we might then guefs at the depth of the caufe 



y 



Gt|ier earthquakes, where we knew their velocity, by 
taking the f depths proportional to thofe velocities 
which probably would anfwer very nearly. * 

10%. Secondly] If, in any inftance, it fhould be 
poffible to know how much the motion of any earth- 
quake was retarded by paffing under the ocean, tho 




Th 



ing up the hills, come toil 
See art. 43. and Fig. 3, 

+ See the note to art. 6^ 



manner 



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69 

depth of the ocean being known, the depth at which, 
the vapour paiTed would be known alfo 5 for the ve- 
locity under the water would be to the velocity, if 
there had been no water, in the fubduplicate ratio of 
the weight in the latter cafe to the weight in the 
former j hence allowing earth to be about two and 
half times the weight of. water, the depth will be 
readily found. . 

103. Thirdly, Let us conceiye the earth to be 
formed according to the idea before given of it, and 
that the fame flrata are at a medium of the fame 
thicknefs for a very great extent, as well in thofe 
places, where feveral of the upper ones are wanting, 
as where they are not. Upon this fuppofitlon, we 
may difcover the depthj at which the' vapour palTes, 
by comparing the feveral velocities of the fame earth-' 
quake in places, where the * thipkneffes of the fuper- 
incumbent mafs are different. ■ It rniill be acknow- 

legcd, indeed, that fuch obfervations with regard ta 

time, as would enable us to determine thefe veloci- 
ties, are in general much too nice to be expedied : 
the matter, however, is not altogether defperate, as 
we may colle(5l them, in fome meafure perhaps, from 
other circumftances, fuch, for inftance, as the degree 
of -]• agitation in different waters, the proportional 
J fuddennefs, with which the earth is lifted in dif- 
ferent places, ^c, 

104. As 



■v 



* In order to know this difference, it will be neceflary to trace 
the thicknefs of thofe ftrata, which are found in fome of the places^ 
but are wanting m others. 
• f See art. 71 and 72. 

X This may be kno^n from the diftance to which the mercury 

fubfides in the barometer, upon the firft railing of the earth by the 



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70 




104. As the obfervations relating to the earthquake 
of the I ft of Noyember 1755 are too grofs, it would 
be in vain to attempt, by any of the foregoing me 



thods, to detejrmine with any certainty the depth at 



which the eaule of it lay'; but, if I might be allowed 
to form a randori guefs about it, I.fhould fuppofe. 



(upon a comparifon of all circumftances) that it could 
not be much lefs than a mile, or a mile and half, and 
I think it is probable, it did not exceed three, miles. 

C O N C L U S ION. 



IOC. Thus have I endeavoured to fhew how the, 
principal phgenomena of earthquakes may be pro- 
duced, by a eaufq with which none, that I ha ve feen , ^ 
appear to me. to be incompatible. As J h^ve not 
knowingly mifrepirefented any fadt, fo neither have I 

defignedly omitted any that appeared to affect the main 

queftion; but, that I might not unneceflarily fwell 

what had already much exceeded the limits at iirft in- 
tended for it, I have omitted,. 



1 06. 




Thofe minuter 




which 



almoft every reader would eaiily account for, from 

what. has been faid already, and which did not feem 
to lead to any thing farther: fuch, for inftaiice,; are 
the fudden flopping and gufhing out of fountains, oc- 

calloned by the opening or contrading of .fiflures^ tha 
dizzinefs and ificknefs people feel, from the almoft 
imperceptible wave-like motion, &c. 



■^-4 



vapour 



:hquake 



omenon, which is a cpnimon 



I of November J755> except at Amfter- 
dam, where the mercury fubfided more than an inch.; See Jiift# 



Philof. of Earthq. p, 309 



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107. Secondfyy Thofe appearances which feemed 
to depend upon particular circumftances, and of 
which, therefore, unlefs we had a more exad: know- 
lege of the countries where they happened, it would 
have been impoffiblc to give any account, without 
having recourfe to uncertain conjectures j of this kind, 
was the greater agitation of the waters in the lakes of 
Switzerland, at the time of the earthquake of the ift 
of November 1 7 5" f, than during the * earthquake of 
the 9th of December following, though the houfes 
upon the borders of them were more violently fhaken 



by the latter. And, 

108. Z^y?/^, Thofe appearances, which only feem 
to have an accidental connedion with earthquakes, or 
the caufes of them 3 of this kind, are the effeds which, 
in fome inftances perhaps, they produce on the wea- 
ther J the diftempers which are fometimes faid to fuc- 

ceedthem; the difturbance which, we are told, they 

the 



■-, 



have fometimes occafioned, during the ihocks 
direction of the magnetic needle, ^c, none of which 
are obferved to be conftant attendants on earthquakes, 

nor do they feem materially to affed the folution given 
either one way or other. 




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