48 A. Milward, Esq., on the Motion of a Lava-Stream. 
cier which moves forward from it will have to that extent a 
higher level than the part which moved forward from the 
lower ridge; and at the same time that the ridge is enlarged, 
it is advanced further down the glacier, because the accumu- 
lation behind is also advanced. It must be remembered, how- 
ever, that the ridge even when thus enlarged is small, com- 
pared with the mass of fallen névé resulting from the gush 
itself. The effects of adding tea-cups full of plaster in one 
of Professor Forbes experiments will illustrate this. The 
addition of each tea-cupful causes a crease, the direct result 
of the accumulation at the head of the slope, and the weight 
of this crease or accumulation may cause a ridge a little be- 
yond it;* but this is so small as not to be remarked. The 
same relative insignificance must be true in the glacier. For 
a short time after a gush the glacier advances from the new 
ridge at a higher level B, but soon the advance is made from 
the accumulation resulting from the gush itself, and is at a 
very much Aigher level C, and forms the main summer ridge. 
Thus each ridge is preceded by a smaller rise, as in fig. 2, 
dependent on the difference of height of the two ridges, 
and is doubtless very small. The distance between the 
gveater and lesser rise is represented by A B C in fig. 1, and 
yet, small as it is, this is all that results from mechanical 
pressure, the main ridge is the direct result of the accumu- 
lation produced by the gush, as was suggested in my former 
* his will depend on the consistency of the plastic mass. 
