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 1864, he communicated another paper to the Philosophical Magazine, in which he modified his views to a certain extent (and made the important admission that it was perhaps impossible to say whether water or ice had produced the greatest amount of erosion), although upon the whole he adhered to his former assertions. This paper contained one remarkable passage; remarkable, because it partly showed the workings of its author's mind, and because it was, apparently, intended to controvert Professor Ramsay's theory. It was as follows:—

"On the higher slopes and plateaus—in the region of cols—the power (of glaciers) is not fully developed; but lower down tributaries unite, erosion is carried on with increased vigour, and the excavation gradually reaches a maximum. Lower still the elevations diminish and the slopes become more gentle; the cutting power gradually relaxes, and finally the eroding agent quits the mountains altogether, and the grand effects which it produced in the earlier portions of its course entirely disappear." —Phil. Mag., Oct. 1864, p. 264.

That is to say, precisely in the situations where Professor Ramsay required glaciers to produce the greatest effects, Dr. Tyndall asserted they produced none whatever! Professor Ramsay did not allow much time to elapse before he contradicted these statements categorically.

"Every physicist," said he, "knows that when such a body as glacier-ice descends a slope, the direct vertical pressure of the ice will be proportional to its thickness and weight and the angle of the slope over which it flows. If the angle be 5°, the weight and erosive power of a given thickness of ice will be so much, if 10° so much less, if 20° less still, till at length, if we may imagine the fall to be over a vertical wall of rock, the pressure against the wall (except accidentally) will be nil. But when the same vast body of ice has reached the 'plain, then motion and erosion would cease, were it not for pressure from behind (excepting what little motion forward and sideways might be due to its own weight). This pressure, however, must have been constant as long as supplies of snow fell on the mountains, and therefore the inert mass in the plain was constantly urged onwards; and because of its vertical pressure its direct erosive power would necessarily be proportional to its thickness, and greater than when it lay on a slope; for it would grate across the rocks, as