CONJECTURE TESTED.

283

westerly one; here, on the contrary, the most westerlystake moves with more than twice the velocity of the mosteasterly one.

To enable me to compare the motion of the eastern andwestern halves of the glacier with greater strictness, myable and laborious companion undertook the task of mea-suring with a surveyor’s chain the line just referred to;noting the pickets which had been fixed along the line, andthe other remarkable objects which it intersected. Thedifficulty of thus directing a chain over crevasses andl’idges can hardly be appreciated except by those whohave tried it. Nevertheless, the task was accomplished, andthe width of the Mer de Glace, at this portion of its course,was found to be 863 yards, or almost exactly half a mile.

Referring to the last table, it will be seen that thetwo stakes numbered 12 and 13 moved with a commonvelocity of 23^ niches per day, and that their motion isswifter than that of any of the others. The point ofswiftest motion may be taken midway between them, andthis point was found by measurement to lie 233 yards westof the dirt which marked the junction of the Glacier duGeant with its fellow tributaries: whereas, in the formercases, it lay a considerable distance east of this limit,Its distance from the eastern side of the glacier was601 yards, and from the western side 262 yards, being170 yards west of the centre of the glacier.

But the measurements enabled me to take the stakes inpairs, and to compare the velocity of a number of themwhich stood at certain distances from the eastern side ofthe valley, with an equal number which stood at the samedistances from the western side. By thus arranging thepoints two by two, I was able to compare the motion ofthe entire body of the ice at the one side of the centralline with that of the ice at the other side. Stake 17stood about as far from the western side of the glacier as