To account for the very great difference of the matters that cover Herculaneum and Pompeii, I have often thought that, in the eruption of 79, the mountain must have been open in more than one place. A passage in Pliny's letter to Tacitus seems to say as much: "Interim e Vesuvio monte pluribus locis latissimae flammae, atque incendia relucebant, quorum fulgor et claritas tenebras noctis pellebat:" so that very probably the matter that covers Pompeii proceeded from a mouth, or crater, much nearer to it than is the great mouth of the Volcano, from whence came the matter that covers Herculaneum. This matter might nevertheless be said to have proceeded from Vesuvius, just as the eruption in the year 1760, which was quite independent of the great crater (being four miles from it), is properly called an eruption of Vesuvius. In the beginning of eruptions, Volcanos frequently throw up water mixed with the ashes. Vesuvius did so in the eruption of 1631, according to the testimony of many contemporary writers. The same circumstance happened in 1669, according to the account of Ignazzio Sorrentino, who, by his history of Mount Vesuvius, printed at Naples in 1734, has shewn himself to have been a very accurate observer of the phaenomena of the Volcano, for many years that he lived at Torre del Greco, situated at the foot of it. At the beginning of the formation of the new mountain, near Puzzole, water was mixed with the ashes thrown up, as will be seen in two very curious and particular accounts of the formation of that mountain, which I shall have the pleasure of communicating to you presently; and in 1755, Etna threw up a quantity of water in the beginning of an eruption, as is mentioned in the letter I sent you last year upon the subject of that magnificent Volcano[32]. Ulloa likewise mentions this circumstance of water attending the eruptions of Volcanos in America. Whenever therefore I find a _tufa_ composed exactly like that which immediately covers Herculaneum, and undoubtedly proceeded from Vesuvius, I conclude such a _tufa_ to have been produced by water mixing with the erupted matter at the time of an explosion occasioned by subterraneous fire; and this observation, I believe, will be of more use than any other, in pointing out those parts of the present _terra firma_, that have been formed by explosion. I am convinced, it has often happened that subterraneous fires and exhalations, after having been pent up and confined for some time, and been the cause of earthquakes, have forced their passage, and in venting themselves formed mountains of the matter that confined them, as you will see was the case near Puzzole in the year 1538, and by evident signs has been so before, in many parts of the neighbourhood of Puzzole; without creating a regular Volcano. The materials of such mountains will have but little appearance of having been produced by fire, to any one unaccustomed to make observations upon the different nature of Volcanos.