1845.] EXAMPLES OF THE VELOCITY OF LAVA STREAMS. 93 



Engineering in Glasgow, and which has been printed in the 

 Philosophical Magazine for March 1845, to which therefore I 

 may refer. I need only state at present that it demonstrates, 

 from observations on the flow of Stockholm pitch with a speed 

 wholly insensible, and which requires some months for its 

 accomplishment even in small masses, that a motion, of the 

 nature of fluid motion, takes place at temperatures at which 

 the pitch remains so hard as to be fragile throughout, and 

 presents angular fragments with a conchoidal fracture. Mr. 

 Gordon adds, that the resistance of the pitch to its own forward 

 motion produces bands of differential velocity, and having the 

 frontal dip. 



EDINBURGH, FEBRUARY 26, 1845. 



NOTE ON THE VELOCITY OF LAVA, REFERRED TO IN PAGE 85. 



The following are a few facts which I have collected on the 

 velocity of lava. That of Vesuvius in 1805 appears to be the 

 most fluid on record. Von Buch, who was in company with 

 MM. de Humboldt and Gay-Lussac, describes it as shooting 

 suddenly before their eyes from top to bottom of the cone in 

 one single instant,* which must correspond to a velocity of 

 many hundred feet in a few seconds without interpreting it 

 literally. Melogrami, quoted by Breislak,f says it described 

 three miles in four minutes, or about seventy-five feet per 

 second at a mean. The same lava, when it reached the level 

 road at Torre del Greco, moved at the rate of only eighteen 

 inches per minute, or three-tenths of an inch per second. J The 

 lava of 1794 (Vesuvius) reached the sea, a distance of 12,961 

 feet, in six hours, or passed over one-third of a mile per hour, 

 or eight inches per second ; whilst the lava of Etna, in 1651, 

 described sixteen miles in twenty-four hours, or above a foot 

 per second the whole way. That of 1669 (Etna), which destroyed 



* Bibliotheque Britaimique, vol. xxx. The vertical height of the cone proper 

 is 700 or 800 feet ; the length of the slope may therefore be 1300 feet, 

 f Institutions Geologiques, iii. 142. 

 | Nicholson's Journal, vol. xii. Breislak, Campanie, i. 203. 



