The comparative rate of advance in the different parts of the glacier was ascertained this summer with greater precision than before. The rows of stakes planted in a straight line across the glacier by Agassiz and Escher de la Linth, in the previous September, now described a crescent with the curve turned toward the terminus of the glacier, showing, contrary to the expectation of Agassiz, that the centre moved faster than the sides. The correspondence of the curve in the stratification with that of the line of stakes confirmed this result. The study of the stratification of the snow was a marked feature of the season's work, and Agassiz believed, as will be seen by a later letter, that he had established this fact of glacial structure beyond a doubt. The origin and mode of formation of the crevasses also especially occupied the observers. On the 7th of August, Agassiz had an opportunity of watching this phenomenon in its initiation. Attracted to a certain spot on the glacier by a commotion among his workmen, he found them alarmed at the singular noises and movements in the ice. "I heard," he says, "at a little distance a sound like the simultaneous discharge of fire-arms; hurrying in the direction of the noise, it was repeated under my feet with a movement like that of a slight earthquake; the ground seemed to shift and give way under me, but now the sound differed from the preceding, and resembled a crumbling of rocks, without, however, any perceptible sinking of the surface. The glacier actually trembled, nevertheless; for a block of granite three feet in diameter, perched on a pedestal two feet high, suddenly fell down. At the same instant a crack opened between my feet and ran rapidly across the glacier in a straight line."* (* Extract from a letter of Louis Agassiz to M. Arago dated from the Hotel des Neuchatelois, Glacier of the Aar, August 7, 1842.) On this occasion Agassiz saw three crevasses formed in an hour and a half, and heard others opening at a greater distance from him. He counted eight new fissures in a space of one hundred and twenty-five feet. The phenomenon continued throughout the evening, and recurred, though with less frequency, during the night. The cracks were narrow, the largest an inch and a half in width, and their great depth was proved by the rapidity with which they drained any standing water in their immediate vicinity. "A boring-hole," says Agassiz, "one hundred and thirty feet deep and six inches in diameter, full of water, was completely emptied in a few minutes, showing that these narrow cracks penetrated to great depths."