ANALOGY OF GLACIERS TO LAVA STREAMS. 
149 
inconceivable degree, and at length, when it ceases to present the slightest external 
trace of fluidity, its movement can only be ascertained by careful and repeated ob- 
servations, just as in the case of a glacier. In November 1843, 1 watched lava issuing 
rapidly from a small mouth in the crater of Vesuvius at the rate of about one foot in 
a second. The eruption of Etna in 1832 advanced at the rate of five miles in two 
days, which is at the rate of one foot in about six seconds* * * § . We may contrast with 
this the eruption of Etna in 1614, which yielded a lava which advanced but two 
miles in ten years according to DoLOMiEU-f-, during the whole of which time its motion 
was sensible. This gives a mean rate of rather more than three feet per day ; but at 
the conclusion it was no doubt much slower. 
Mr. ScropeJ saw the lava of 1819 in the Val del Bove moving down a consider- 
able slope at the rate of a yard a day, nine months after its eruption. It had, he 
adds, the appearance of a huge heap of rough cinders ; its progression was marked 
by a crackling noise due to friction and straining, and “on the whole was fitted to 
produce any other idea than that of fluidity. In fact,” he continues, “ we must 
represent to ourselves the mode in which the crystalline particles of lava move 
amongst one another, rather as a sliding or slipping of their plane surfaces over each 
other, facilitated by the intervention of the elastic (?) fluid, than as the rotatory 
movement which actuates the molecules of most other liquids.” It is generally con- 
formable to this view that we find in Hamilton’s Campi Phlegrcei (fol. 1. 38. Note ) the 
curious remark that some lava is so incoherent, or whilst fluid has so little viscosity, 
that in issuing from the volcano (Vesuvius) it has appeared “ farinaceous, the particles 
separating as they forced their way out, just like meal coming from under the grind- 
stones.” 
From all this it is quite clear that the seeming rapidity of the parts of a glacier, 
or the slowness of its motion, cannot be taken as the slightest evidence of its moving 
otherwise than as a fluid, contending with the rigor of the parts which include and 
resist the moving force, which is truly hydrostatic though limited in its exercise. 
It is manifestly futile and unphilosophieal to seek one cause of motion in a lava 
which, like that of Vesuvius in 1805, must have described as many hundred feet in a 
minute as that of 1614 from Etna probably did in a year for the mean daily 
motion of the latter during ten years was three feet ; but toward the end of that time 
it piust evidently have had for a long period an average motion of one-half or one- 
quarter of this, and therefore below the observed mean movements of certain glaciers. 
Fluidity, in the first instance as in the second, was the propelling vehicle or manner in 
* E. de Beaumont, Recherches sur le Mont Etna. 
f Quoted by E. de Beaumont, p. 85. The original is in the Journal de Physique, vol. i. of the New Series, 
where it is mentioned that the same slowness of motion has been observed in lavas of Vesuvius. Ferrara 
(Descrizione del Etna. Palermo, 1818) denies this statement, but not I think on sufficient grounds. 
I On Volcanoes, p. 102. 
§ See Note on the Velocity of Lava Streams at the end of this paper. 
