86 VISCOUS THEORY OF GLACIER MOTION. [1845. 



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 grindstones." .- 



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 unphilosophical 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 must 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 which gravity acted, and 

 this is a sufficient answer to any attempt to maintain that the 

 plasticity of a glacier is a collateral but not a primary cause of 

 motion a distinction surely without a difference. 



As in the case of all imperfect fluids, the central and super- 

 ficial particles move faster than the lateral and inferior ones ; 

 and when the fluidity is exceedingly imperfect, as in those long- 

 flowing lavas, there must be a rupture of continuity between 

 the parts to permit them to slide and jostle past one another. 

 This is evidently the cause of the noise referred to by Mr. 

 Scrope and other writers. This tearing up of the stream into 

 longitudinal stripes, occasioned by the varying velocity of the 

 parts, is thus described by M. Dufrenoy in his account of Vesu- 

 vius : " La plupart des coulees presentent des bandes longitu- 

 dinales assez paralleles entre elles : ces larges stries saillantes 



* See Note on the Velocity of Lava Streams at the end of this paper. 



