S' 



VOL. LXX.] PHILOSOPHICAL TRANSACTIONS. 6l3 



IV. On the Eruption of Mount Fesuvms in August, l77Q. In a Letter from 

 Sir JVm. Hamilton, K. B., F. R. S. p. 42. 



Since the eruption of 1767, Vesuvius has never been free from smoke, nor 

 ever many months without throwing up red-hot scoriae, which, increasing to a 

 certain degree, were usually followed by a current of liquid lava, and except in 

 the eruption of 1777, those lavas broke out nearly from the same spot, and ran 

 much in the same direction, as that of the famous eruption of 1767. No less 

 than 9 such eruptions are recorded since that great one, and some of them were 

 considerable. The lavas, when they either boiled over the crater, or broke out 

 from the conical parts of the volcano, constantly formed channels as regular as 

 if they had been cut by art down the steep part of the mountain, and, while in 

 a state of perfect fusion, continued their course in those channels, which were 

 sometimes full to the brim, and at other times more or less so, according to the 

 quantity of matter in motion. 



These channels, on examination after an eruption, are found to be in general 

 from 2 to 5 or 6 feet wide, and 7 or 8 feet deep. They are often hid from the 

 sight by a quantity of scoriae forming a crust over them, and the lava having 

 been conveyed in a covered way for some yards, come out fresh again into an 

 open channel. After an eruption Sir W. had walked in some of those subter- 

 raneous or covered galleries, which were exceedingly curious, the sides, top, and 

 bottom, being worn perfectly smooth and even in most parts by the violence of 

 the currents of the red-hot lavas, which they had conveyed for many weeks 

 uccessively; in others, the lava had incrusted the sides of those channels with 

 some very extraordinary scoriae: beautifully ramified white salts, in the form of 

 dropping stalactites, were also attached to many parts of the ceiling of those 

 galleries. It is imagined here, that the salts of Vesuvius are chiefly ammoniac, 

 though often tinged with green, deep, or pale yellow, by the vapour of various 

 minerals. 



In the month of May last, (1779), there was a considerable eruption of 

 Mount Vesuvius, when Sir W. passed a night on the mountain in the company 

 of Mr. Bowdler, of Bath, as eager as himself in the pursuit of this branch of 

 natural history. They saw the operation of the lava, in the channels as above- 

 mentioned, in the greatest perfection. After the lava had quitted its regular 

 channels, it spread itself in the valley, and, being loaded with scoriae, ran 

 gently on, like a river that had been frozen, and had masses of ice floating 

 on it: the wind changing when they were close to this gentle stream of lava, 

 which might be about 50 or 60 feet in breadth, incommoded them so much 

 with its heat and smoke, that they must have returned without having satisfied 

 their curiosity, had not the guide proposed the expedient of walking across it. 



