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dified , according to the different contextures and forms of 
buildings, asforinftance in arched fabricks the trembling may be 
more uniformly vibrating, and the found more confiderable, as 
the air included is capable of more undulations, which I pre- 
fumemay be thereafon why you heard it foloud in the Laborato- 
rj'y for it is certain that all other founds have a great advantage 
there. 
7 But thofe that were abroad in the fields and open air, percei- 
ved, with a gentle fhaking, a hollow murmur towards the furface 
of the earth, not unfitly compared to the groaning of fome planks 
of Elm, A(h, or Firr when the application of fire, to the wood by 
rarefacfiion excites a motion in the air and watry parts contained 
in the caverns or- air veflTels, lying like fo many little wind pipes 
(vi^. Dr. Grews Anatomy of ^Plants li. 3. c. 7.§. 7. ) To as to Ihake 
the ligneous fiibres, and caufe both a trembling, and found: the 
comparifon, tho feemingly trivial, may be more confiderable, than 
he is aware, that ftialldefpife it, if he make an allowance for the 
different quantity and contexture betwixt the board and the earth, 
together with the various application of the heat, which in the one 
cafe is outward, and -in the other is fuppofedto be internal. 
Now that that there is confiderable heat within the earth is ma- 
nifeft, from the experience of Miners working in the deeper 
grooves, of which you may read more in Mr. Bojles treatife of [tih- 
terrmed qudlties', from thofe hot fprings which breakout thence, 
from firmentations occafioned by mineral fpirits, and to give a 
vulgar inftance from the mixture of quicklime and water, together 
with feVeral ebullitions in diverfe exferements too commonly 
known, and too many to be here inferted. Nor is it lefs commonly 
obfer ved,that fuch heats and fermentations within the earth are aug- 
mented by frofty weather, when the fleams being more pent up, 
and hindred from breaking out, do work more forcibly upon one 
another, as 1 have hinted before. 
And that founds and tremblings may be produced by fuch heat, 
though it did but work upon ^/>,, watery vafours^ or nitre only in- 
cluded in pores and cavityes, appears by feveral exferimentSj as that 
of fillingglafs bubbles half full with water and nitrey which being 
fet to the heat of the fire will tremble with a fort of hnmming 
found, and after that break with a great deal of noife and vio- 
lence; but thofe vulgar obfervations, of greenwood burning, or 
water boyling in a covered vefTel, may be fufRcient for our prefent 
purpofe. 
8 By whaL^nalogy this of ours may be bed explained 1 (hall not 
yet 
