On the 10th of October 1720, near the island Tercera, a very considerable fire arose out of the sea; some mariners were sent by the order of the governor to take a view of it, and who having come near it, perceived, on the 19th of the same month, an island which appeared only as fire and smoke, with a prodigious quantity of ashes thrown to a distance, as if caused by the force of a volcano, with a report like that of thunder. An earthquake happened at the same time, which was felt in the circumjacent places, and great quantities of pumice-stones were observed floating on the sea around the new island; pumice-stones indeed have sometimes been seen swimming in the midst of the high seas.[AE]
[AE] See Phil. Trans. Abridg. vol. VI. part ii. page 254.
The historian of the academy, anno 1721, says on this event, that after an earthquake in the island of St. Michael, one of the Azores, there appeared between this island and Tercera a torrent of fire, which gave birth to two new shoals; and the next year he gave the following detail:
"M. de l'Isle has informed the academy of many particulars concerning the new island among the Azores, which he received in a letter from M. de Montagnac, consul at Lisbon.
"Being in a vessel, which was moored the 18th of September 1721, before the fortress of the town of St. Michael, M. de Montagnac learnt the following account from the pilot:
"On the 7th of December 1720, at night, there was a great earthquake in Tercera and St. Michael, which are about 18 leagues apart, and between which a new island sprung up: it was remarked at the same time, that the point of the island of Peak, 30 leagues distant, and which before threw out fire, was sunk and emitted none; but the new island kept throwing out a constant thick smoke, and which I plainly perceived from the vessel I was in. The pilot assured us that he had gone round the island, rowing as near it as he conceived to be safe. On the south side he threw a line of sixty fathoms without finding any bottom; on the west side the water was greatly changed, appearing white, blue and green, and which extended two thirds of a league, where it seemed ready to boil. On the north-west, the part from which the smoke issued, he found, at 15 fathoms, a bottom of thick sand; he threw a stone in the sea, and where it fell the water seemed to boil and bubble with impetuosity; the bottom was so hot that it twice melted some grease fastened at the end of the sounding line. The pilot observed also on that side that smoke issued from a small lake bounded by a sand bank. This island is almost round and high enough to be perceived at the distance of seven or eight leagues in clear weather.
"It has since been learnt from a letter of M. Adrian, French consul in the island of St. Michael, dated March 1722, that the new island had considerably diminished, that it was almost level with the water, and there was every appearance it would not last long."
It is therefore by these, and a great number of other facts of a similar nature, very evident that inflammable matters are enclosed in the earth under the bottom of the sea, and that they sometimes cause violent explosions. The places where this happens might be termed marine volcanos, and which differ from common volcanos only by the shortness of the duration of their effects; for the fire having opened itself a passage, the water must penetrate therein and extinguish it. The elevation of new islands must consequently leave a void space which the water would shortly occupy, and this new earth, which is only composed of matters thrown out by the marine volcano, must resemble that of Monti di Cinere and other eminencies which terrestrial volcanos have formed. Now as the water rushes in, during the violence of the explosion, and fills the vacancies that it occasions, that is clearly the reason why these marine volcanos act less frequently than other volcanos, although the causes of both are the same.
These subterraneous, or sub-marine fires are doubtless the cause of all those ebullitions of the sea, which sailors have remarked in various places, and as well as of the water-spouts we have before mentioned; they likewise produce storms and earthquakes, which are not less felt on the sea than on the land. Islands formed by these sub-marine volcanos, are generally composed of pumice-stone, and calcined rocks, and produce, like those of the land, violent earthquakes and commotions.