310 Charles Maclaren, Esq., on the 



on the top of Vesuvius, flowed sluggishly, according to my estimate, 

 at the rate of a foot in five or six seconds, and had morsels of solid 

 lava floating on it. In the eruption of 1831, Mr Auldjo found the 

 lava the day after its eruption advancing in the low ground at the 

 slow rate of ten feet per hour. The coulees of Etna are on a grander 

 scale. Mr Scrope saw one " slowly progressing at the rate of about 

 a yard per day" nine months after it had issued from the flank of 

 the mountain ; and other currents are described by Ferrara and Dolo- 

 mein " as still moving on ten years after their emission," — clear 

 evidence of a pasty condition and very slow motion like that of a glacier. 

 The pasty condition which lava assumes is further exemplified in the 

 stringy forms and strange shapes into which it is drawn out or twisted, 

 resembling coils of rope, horns, festoons, &c, and still better, perhaps, 

 in the multitudeof cells it contains, curiously elongated in the direction 

 of its motion. Mr Scrope applies to it the terms " viscous, glutinous, 

 ductile, semi-solid." — {Considerations on Volanoes,ip. 102.) Again, 

 there is a similarity even in the external form of the glacier and the 

 lava coulee. The latter moves on between two ridges of seorise, or 

 solidified portions of its own substance, as a glacier advances between 

 lines of fragments torn from the rocks it has been in contact with. 

 Both are resisted by friction on the sides and bottom of their 

 channels ; in both, owing to this resistance, the middle moves faster 

 than the sides and bottom, and the upper surface is raised into a con- 

 vex form. Further, the parallel flutings {cannelures) noticed on the 

 surface by M. de Beaumont, are the counterparts of Forbes's " blue 

 bands,'' and like them arise from the different parts of the current or 

 coulee, moving with different velocities. In short, widely unlike as 

 the substances are, there is no doubt that gravitation acts upon 

 them very nearly in the same manner, and that if a mean slope of 

 30', or one foot in 114, suffices to carry a deep coulee of lava over 

 a line of 50 miles, there is a reasonable presumption that, with a 

 declivity equally small, a glacier 2500 feet deep might advance from 

 Martigny to Chasseron. yiev on svjsd 



Professor Forbes, an excellent authority on such questions, con- 

 siders it certain that the law which regulates the motion of the more 

 perfect fluids, such as water, is applicable to the more imperfect, such 

 as glacier ice {Travels in the Alps, p. 385, first Ed.) The effect of 

 that law, in reference to the dimensions of a stream, is thus concisely 

 enunciated — " A stream of twice the length, breadth, and depth of 

 another, will flow on a declivity half as great, and one of ten times 

 the dimensions upon one-tenth of the slope." Now, the mean slope 

 of the Glacier de Bois over a space of three miles where it was most 

 level, was found by the same author to be 4| degrees, or about 1 foot 

 in 13 ; its depth, near the upper limit of that space, was reported to 

 be 350 feet; but this was believed to be the extreme depth, and the 

 mean for the three miles may perhaps be taken at 250 feet. Its 

 ra£q of motion \:ui«>- from day to day- small in winter, greatest in 



