which also contains other ice crystals, and water at 0 deg.  C., then the stressed crystal will melt and become water, but its counterpart or equivalent quantity of ice will reappear elsewhere in the vessel.  This is, obviously, but a deduction from the principles we have been examining.  The phenomenon is commonly called “regelation.”  I have already made the usual regelation experiment before you when I compressed broken ice in this mould.  The result was a clear, hard and almost flawless lens of ice.  Now in this operation we must figure to ourselves the pieces of ice when pressed against one another melting away where compressed, and the water produced escaping into the spaces between the fragments, and there solidifying in virtue of its temperature being below the freezing point of unstressed water.  The final result is the uniform lens of ice.  The same process goes on in a less perfect manner when you make—­or shall I better say—­when you made snowballs.

We now come to theories of glacier motion; of which there are two.  The one refers it mainly to regelation; the other to a real viscosity of the ice.

The late J. C. M’Connel established the fact that ice possesses viscosity; that is, it will slowly yield and change its shape under long continued stresses.  His observations, indeed, raise a difficulty in applying this viscosity to explain glacier motion, for he showed that an ice crystal is only viscous in a certain structural

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direction.  A complex mixture of crystals such, as we know glacier ice to be, ought, we would imagine, to display a nett or resultant rigidity.  A mass of glacier ice when distorted by application of a force must, however, undergo precisely the transformations which took place in forming the lens from the fragments of ice.  In fact, regelation will confer upon it all the appearance of viscosity.

Let us picture to ourselves a glacier pressing its enormous mass down a Swiss valley.  At any point suppose it to be hindered in its downward path by a rocky obstacle.  At that point the ice turns to water just as it does beneath the skate.  The cold water escapes and solidifies elsewhere.  But note this, only where there is freedom from pressure.  In escaping, it carries away its latent heat of liquefaction, and this we must assume, is lost to the region of ice lately under pressure.  This region will, however, again warm up by conduction of heat from the surrounding ice, or by the circulation of water from the suxface.  Meanwhile, the pressure at that point has been relieved.  The mechanical resistance is transferred elsewhere.  At this new point there is again melting and relief of pressure.  In this manner the glacier may be supposed to move down.  There is continual flux of conducted heat and converted latent heat, hither and thither, to and from the points of resistance.  The final motion of the whole mass is necessarily slow; a few feet in the day or, in winter,