34 
repoet— 1847. 
moved from each other, much more generally than superficial appearance* 
may seem to indicate. The hypothesis that the thicknoss of tho lolid orust 
is extremely small would seem essential to give any degree of probahilitv w 
the first of these views, but I sliall shortly ofiur reasons for helieving such aa 
hypothesis to be untenable ; and mdi*pen«lently of this condderatiM, it t» 
manifest that if that view of the subject were correet, the upper lurface of 
the fluid mass of every volcano not in a state of eruption ma.-l be a portvoD 
of the same surface of e<jual pressure, and consequently at the same heigut 
above the level of the st;a; nor would it seem to be possible for an erophoa 
to take place at a great elevation in one volcano, without producing sirnul* 
taneous eruptions at least from neighbouring volcanos situated at lower lew*- 
The same remarks would be applicable to any local group of voloancn fretlj 
coninmnicaiing with each other, except that the common level of the *uc 
faces of the volcanic fluid in one sucli group might be difl'erent from that of 
another, the communication being independent of a common fluid miclcu*- 
It does not appear, however, that we have the slightest evidence of « 
equality of altitude of the incandcNceiit intt>« of neighbouring volcano*, a™ 
it is certain that there is no synchronism in the epochs of their eruption*" 
The groat rruplions of Etna have not been accompanioil by sitnuUaneou* 
eruptions of Vesuvius, while the intermediate volcano of Stromboli ha* P*]®* 
served its comjinrative tranquillity during the most violent disturbances of iW 
neighbours, and renmiiieU without a single recorded eruption during '2000 
years; and possibly this comparative repose may have been of much longw 
duration. 
Such facts as these would seem to prove beyond ilonbt that the fluid on* 
clei of different volcanos have generally no perfectly free conmmnicatioD 
with each other, and may so far, in tlieir actual state, be regarded as iusulawd 
phsnomcna. In what degree this insulatiun may have originally existed, or 
how far it may have been the result of physical causes acting during bn? 
perio<ls of time, are questions which wo slmll have hereafter t<J discuss. 
3. M itli reftnmee to the gases above nteiitioncd, we may remark that v« 
aic not here concerned in any degrt'e with their chemical nature, but merely 
with their property ot elasticity. Distinct evidence of the continuous 
ration of gases in the interior of the volcanic mass, and their 
contimiou-i ascent to the surface, is afforded by such volcanos as iliat o' 
btromboli and Kiruuca*, w-hich remain for long periods of time In sensibly 
the same »lute. uninterriqjtml by those periodical explosive eruptions which 
form the moat striking feature in volcanic action. The fluid masses in lhe« 
volcanos remain at nearly a constant elevation, and are observed to be i" « 
constant state of comparatively quiet ebiilliiion, arising, undoubtedly, from the 
ol elastic vapours formed within them. The fact 
‘/‘I'® volcanic mass is due to heat is universally 
f n " ith apparent reason, whether it be 
ciscly of the same nature us the fluidity of a perfect fluid, or that of an ordi* 
P- tldrvnlolm^Twella^inth 
donee n^pectL, S po.Ui^n . f consequentlr afford iniportant ^ 
of tiu! ocean. Volcanos in iri»ni.,-ni ^ 'laid " hh reference to tli* |r 
omporaiive rqrose whioh i , of tlds kind during the ]wno.is 
the l^el of the^a "“ervem; between their eruptions. Kirauca is WOO feet #'J0' 
