VOLCANO. 



the edge of the crater, which he perceived funk in fomc 

 fluid when they reached the bottom. This fluid could not 

 be aqueous, fince it would foon have been evaporated by ex- 

 ccfllve heat ; lie fuppofed it to be melted fulphur, as lie faw 

 that fubftance trickle down the fides, againll which it had 

 fubliraed. With a good telefcope he could difcover at the 

 bottom two fmall pools, which he fuppofed to be full of 

 the fame combuftible matter. He likewife obferved, that 

 the fumes which in the dav-time appeared white, were by 

 night fplendent, but placid flames, th,it rofe above the 

 mountain, and diffufed their light to fome diftaiice. 



Spallanzaiii, who vifited Vulcano feven years after Dolo- 

 mieu, found the bottom only about a quarter of a mile 

 deep, but intolerably hot. 



The changes which took place in this interval, were pro- 

 bably occafioned by a violent commotion which occurred in 

 the month of March 1786, during which the crater threw 

 out a prodigious quantity of volcanic powder or fand with 

 immenfe volumes of fmoke and flame. This eruption lalled 

 fifteen days. 



That the bottom of the crater fliould vary confiderably in 

 depth after every eruption will not appear furprifmg, if we 

 refleft that this bottom is a criift of congealed lava, more 

 or lefs covered with loofe materials, which have fallen upon 

 it. When the lava which has been forced up near to the 

 brink of the crater, remains ftationary at the clofe of an 

 eruption, and folidifies, the melted lava will gradually fink 

 down as the intcnCty of the volcanic fire diminifties at the 

 furface, thus leaving a cruft of greater or lefs thicknefs over 

 a hollow fpace below. The depth of this floor from the 

 brink will depend on the quantity of lava which remains in 

 the crater towards the end of an eruption. 



The phenomena preceding and attending volcanic erup- 

 tions, vary according to the fituation in which they break 

 forth, and the magnitude or iiitenfity of the volcanic fire. 

 An eruption may proceed from ancient volcanoes, v\'hich 

 have been dormant for a longer or fliorter period, or it may 

 break out from a new opening or from under the fea. The 

 phenomena moll common to each of thefe fituations we fhall 

 briefly defcribe. The indications of an approaching erup- 

 tion from a dormant volcano, are the increafe of fmoke 

 from the fummit of the crater, which fometimes rifes to a 

 vaft. iieight, branching in the form of a pine-tree. This was 

 the cafe in the memorable eruption of Veiuvius, dtfcribed 

 by Pliny, in tlie year 79 of the Chriftian era. The caufe of 

 tliis appearance is probably the violent efcape of elailic gas 

 driving up the volatile materials into the higher regions of 

 the atmofphere, which in their defcent float at difl'crent 

 heights, according to their fpecific gravity, the heavicll 

 llratum floating over a larger fpace. Tremendous explo- 

 fions, like the firing of artillery, commence after the in- 

 creafe of fmoke, accompanied with tremors of the earth, 

 more or lefs violent, and by eruptions of red-coloured flame 

 and ftones from the crater ; after which, in moll violent erup- 

 tion;!, currents of melted (lone, called lava, flow either over 

 the brink of the crater, or break through the fides of the 

 mountain. Thefe currents, when confolidated by cooling, 

 frequently form a llratum thirty or forty miles in length, 

 feveral miles broad, and feveral yards thick, equalling in 

 extent any continuous llratum, among the regular forma- 

 tions of fecondary flrata. The eruption of lava has been 

 known to continue for feveral months. Black clouds, com- 

 pofed of dark-coloured fand or powder, improperly called 

 aflies, are thrown out of the crater after the lava ceafes to 

 flow. During one eruption of Etna, a fpace of one hun- 

 dred and fifty fquare miles was covered with this fand twelve 

 feet thick. Stones or globiform mafles of melted lava are 



Vol. XXXVII. 



thrown out at the fame time, and fall at a greater or lefs 

 dillance, according to their fize, and the force with which 

 they are ejefted, the larger maffes faUing neareft to the 

 mouth of the volcano. The fmoke and vapour are highly 

 eleftrical, and vivid violent flaflies of lightning dart from it, 

 which frequently occafion much mifchief. Towards the 

 conclufion of the eruption, the colour of the volcanic fand 

 changes to white ; it confiils of pumice in a finely commi- 

 nuted ftate. It is obferved, that when the lava flows freely, 

 the tremors of the earth and the explofion become lefs fre- 

 quent, which proves that they were occafioned by the con- 

 finement of the gafeous and folid matter that is afterwards 

 difcharged. 



Mod of the phenomena here mentioned occur in the 

 eruptions of mount Vefuvius, near Naples. The firfl erup- 

 tion of this mountain recorded in hillory, is that whicii 

 happened in the time of Vefpafian, A.D. 79 ; on which 

 occafion, fays Dion Caffius, great quantities of aflies and 

 fulphureous fmoke were carried not only to Rome, but alfo 

 beyond the Mediterranean, into Africa, and even to Egypt. 

 Birds were fuffocated in the air, and fell down dead upon 

 the ground, and fifhes perifhed in the neighbouring waters, 

 which were made hot, and infected by it. Sir William 

 Hamilton reckons, that the eruption in 1767 was the twenty- 

 leventh from that in the time of Titus. Since 1767 the 

 eruptions have been frequent. 



Bifliop Berkeley has given a particular account of the 

 eruption in 1717 ; for which, fee Phil. Tranf. N° ^54. 

 p. 708, or the Life of Berkeley, in the Biographia Bri- 

 tannica, by Dr. Kippis. 



We have an account of mount Vefuvius, and of the 

 eruption from it in 1737, by the prince of Caflano, in the 

 Philofophical TranfaCtions, N°435. feft. i, 2. 



The matter thrown out flowed like melted lead, and 

 moved about half a mile in an hour, which was then con- 

 fidered as an unnfual velocity. The trees touched by this 

 matter, immediately took fire, and fell. Glafs in houfes 

 was melted into a paft.e. 



Sir William Hamilton has given an accurate and circum- 

 llantial defcription of the eruptions in 1766, 1767, and 

 1779. See Phil. Tranf. vol. Ivii. p. 192, vol. Iviii. p. i, &c. 

 vol. lix. p. i8, &c. vol. Ixx. part i. p. 42, &c. We rtiall 

 feleA his account of the latter. During the whole month 

 of .July the mountain continued in a ftate of fermentation. 

 Subterraneous explofions and rumbhng noifcs were heard, 

 quantities of fmoke were thrown up with great violence, 

 fometimes with red-hot lloncs, fcorix, and alhcs ; and to- 

 wards the er.d of the month thefe fymptoms increafed to 

 fuch a degree, as to exhibit in the night-time the moft beau- 

 tiful fire-works tliat can be imagined. 



On Thnrfday, the 5th of Auguft, the volcano appeared 

 moft violently agitated ; a while and fulphureous fmoke 

 ifTued continually and impetuouily from its cr.iter, one puff 

 feeming to impel another, fo that a m;'.f3 of them was iooa 

 accumulated, to appearance four times the height and fize 

 of the volcano itfelf. Thefe clouds of fmoke were exceed- 

 ingly white, fo tliat the whole refembled an immenfe accu- 

 mulation of bales of the whiteft cotton. In the midft of 

 this very white fmoke, vaft quantities of ftones, fcorite, and 

 aflies were thrown up to the height of two thoufand feet, 

 and a quantity of liquid lava, fecmingly very heavy, wa;. 

 lifted up juft high enough to clear the rim of the crater, 

 and take its way down the fides of tlie mountain. This 

 lava having run violently for fome hours, fuddcnly ceafed, 

 juft before it had reached the cultivated parts of the moun- 

 tain, near four miles from the fpot whence it in"ued. Th«? 

 heat all this day was intolerable at the towne of Somnia and 

 3 K Ottaianu, 



