FIRST ATTEMPT AT MEASUREMENT.

289

ICE-WALL AT THE TACUL.

VELOCITIES OF TOP AND BOTTOM.

( 11 .)

As regards the motion of the surface of a glacier, two lawsare to be borne in mind: 1st, that regarding the quickermovement of the centre ; 2nd, that regarding the locusof the point of maximum motion. Our next care mustbe to compare the motion of the surface of a glacierwith the motion of those parts which lie near its bed.Rendu first surmised that the bottom of the glacier wasretarded by friction, and both Professor Forbes * and M.Martins! have confirmed the conjecture. Theirs are theonly observations which we possess upon the subject; andI was particularly desirous to instruct myself upon thisimportant head by measurements of my own.

During the summer of 1857 the eastern side of theGlacier du Geant, near the Tacul, exposed a nearly ver-tical precipice of ice, measuring 140 feet from top tobottom. I requested Mr. Hirst to fix two stakes in thesame vertical plane, one at the top of the precipiceand one near the bottom. This he did upon the 3rd ofAugust, and on the 5th I accompanied him to measurethe progress of the stakes. On the summit of the pre-cipice, and running along it, was the lateral moraine ofthe glacier. The day was warm and the ice liquefyingrapidly, so that the boulders and ddbris, deprived inces-santly of their support, came in frequent leaps and rushesdown the precipice. Into this peril my guide was about

* * Edinb. Phil. Journ.,’ Oct. 1846, p. 417.t Agassiz, ‘ Syst&me Glaciaire,’ p. 522.

O