As it was necessary, in view of his special programme of work, to penetrate below the surface of the glacier, and reach, if possible, its point of contact with the valley bottom, Agassiz had caused a larger boring apparatus than had been used before, to be transported to the old site on the Aar glacier. The results of these experiments are incorporated in the "Systeme Glaciaire," published in 1846, with twenty-four folio plates and two maps. They were of the highest interest with reference to the internal structure and temperature of the ice and the penetrability of its mass, pervious throughout, as it proved, to air and water. On one occasion the boring-rod, having been driven to a depth of one hundred and ten feet, dropped suddenly two feet lower, showing that it had passed through an open space hidden in the depth of the ice. The release of air-bubbles at the same time gave evidence that this glacial cave, so suddenly broken in upon, was not hermetically sealed to atmospheric influences from without. Agassiz was not satisfied with the report of his instruments from these unknown regions. He determined to be lowered into one of the so-called wells in the glacier, and thus to visit its interior in person. For this purpose he was obliged to turn aside the stream which flowed into the well into a new bed which he caused to be dug for it. This done, he had a strong tripod erected over the opening, and, seated upon a board firmly attached by ropes, he was then let down into the well, his friend Escher lying flat on the edge of the precipice, to direct the descent and listen for any warning cry. Agassiz especially desired to ascertain how far the laminated or ribboned structure of the ice (the so-called blue bands) penetrated the mass of the glacier. This feature of the glacier had been observed and described by M. Guyot (see page 292), but Mr. Forbes had called especial attention to it, as in his belief connected with the internal conditions of the glacier. It was agreed, as Agassiz bade farewell to his friends on this curious voyage of discovery, that he should be allowed to descend until he called out that they were to lift him. He was lowered successfully and without accident to a depth of eighty feet. There he encountered an unforeseen difficulty in a wall of ice which divided the well into two compartments. He tried first the larger one, but finding it split again into several narrow tunnels, he caused himself to be raised sufficiently to enter the smaller, and again proceeded on his downward course without meeting any obstacle. Wholly engrossed in watching the blue bands, still visible in the glittering walls of ice, he was only aroused to the presence of approaching danger by the sudden plunge of his feet into water. His first shout of distress was misunderstood, and his friends lowered him into the ice-cold gulf instead of raising him. The second cry was effectual, and he was drawn up, though not without great difficulty, from a