284 
PEOFESSOE TYNDALL ON THE VEINED STELCTLEE OF GLACLEES. 
with a neutral axis between, the mechanical conditions of the mass being shown by its 
action on polarized light. The same is true of any other substance,— the concave surface 
of the bent prism is compressed. Now at the bases of steep glacier slopes, where the 
inclination suddenly changes, we have a case of this bending, and along with it a thrust 
of the mass behind. The concave surface is turned towards us, and that surface is 
thrown into a state of compression corresponding to the thrust, and to the change of 
inclination. Hence it occurred to me, that the bases of the ice-falls, where the requisite 
change of inclination occurs, were likely to be the manufactories of the transverse 
structure. The experience of 1858 completely verified this idea. 
In illustration of my position I will take a representative case ; and to render my 
observations capable of being easily checked, I will choose one of the most accessible 
glaciers in the Alps,— the lower glacier of Grindelwald. One portion of this glacier 
descends from the Viescherhorner ; but there is another branch which descends from the 
Schreckhorn, Finsteraarhorn and Strahleck, and it is to this latter branch that I now 
wish to direct attention. 
Walking up this glacier from its place of junction with the tributary from the 
Viescherhorner, we come at length to the base of an ice-fall which forbids further 
advance upon the ice. Let the glacier be here forsaken, and let the flanking mountain 
side, either right or left, be ascended, until a position is attained which affords a com- 
plete view of the fall and of the glacier stretching downwards Horn the base of the fall. 
The view from such a position will furnish a key to the development of the transverse 
structure. 
It is, in point of fact, a grand experiment which Nature here submits to our inspec- 
tion. The glacier, descending from its neve, reaches the summit of the fall and is 
broken transversely as it crosses the brow. It descends the fall as a succession ot 
broken clifiy ridges, with transverse hollows between them. In these latter the ice 
debris and the dirt collect, partially choking up the fissures formed in the first instance. 
Carrying the eye downwards along the fall, we see, as we approach the base, these sharp 
ridges toned down, and a little below the base they dwindle into rounded protuberances 
which sweep, in curves, across the glacier. At the centre of the fall there is not a tiace 
of the true structure to be observed. At the base of the fall it begins to appear, ^-at 
first feebly, but soon becomes more pronounced; until finally, at a short distance below 
the fall, the eye can follow the structural groovings right across the smfface of the 
glacier, while the mass underneath has become correspondingly lammated in the most 
beautiful manner. 
It is difficult to convey, by writing, the force of the ewdence which the actual obser- 
vation of this great experiment places before the mind. The ice at the base of the fall 
has to bear the powerful thrust of the descending mass ; but more than this, the sudden 
change of inclination which it suffers throws its upper portion into a state of Holeiit 
longitudinal compression. The protuberances are squeezed more closely together, the 
