At any rate, all reasonable grounds for distrusting the accuracy of Helland's estimates seem to have been removed by later investigations. According to my own observations in the summer of 1886 upon the Muir Glacier, Alaska, the central portions, a mile back from the front of that vast ice-current, were moving from sixty-five to seventy feet per day. These observations were taken with a sextant upon pinnacles of ice recognizable from a baseline established upon the shore. It is fair to add, however, that during the summer of 1890 Professor H. F. Reid attempted to measure the motion of the same glacier by methods promising greater accuracy than could be obtained by mine. He endeavoured to plant, after the method of Tyndall, a line of stakes across the ice-current. But with his utmost efforts, working inwards from both sides, he was unable to accomplish his purpose, and so left unmeasured a quarter of a mile or more of the most rapidly-moving portion of the glacier. His results, therefore, of ten feet per day in the most rapidly-moving portion observed cannot discredit my own observations on a portion of the stream inaccessible by his method. A quarter of a mile in width near the centre of so vast a glacier gives ample opportunity for a much greater rate of motion than that observed by Professor Reid. Especially may this be true in view of Tyndall's suggestion that the contour of the bottom over which the ice flows may greatly affect the rate in certain places. A sudden deepening of the channel may affect the motion of ice in a glacier as much as it does that of water in a river. Other observations also amply sustain the conclusions of Helland. As already stated, the Danish surveying party under Steenstrup, after several years' work upon the southwestern coast of Greenland, have ascertained that the numerous glaciers coming down to the sea in that region and furnishing the icebergs incessantly floating down Baffin's Bay, move at a rate of from thirty to fifty feet per day, while Lieutenants Ryder and Bloch, of the Danish Navy, who spent the year 1887 in exploring the coast in the vicinity of Upernavik, about latitude 73°, found that the great glacier entering the fiord east of the village had a velocity of ninety-nine feet per day during the month of August.[AN] [Footnote AN: Nature, December 29, 1887.] It is easier to establish the fact of glacial motion than to explain how the motion takes place, for ice seems to be as brittle as glass. This, however, is true of it only when compelled suddenly to change its form. When subjected to slow and long-continued pressure it gradually yet readily yields, and takes on new forms. From this capacity of ice, it has come to be regarded by some as a really viscous substance, like tar or cooling lava, and upon that theory Professor Forbes endeavours to explain all glacial movement.