Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/647

Rh GLACIER views with his usual precision :—“These frozen masses, carried along by the slope of the bed on which tl1ey rest, disengaged by the water (arising from their fusion owing to the natural heat of the earth) from the adhesion which they might otherwise contract to the bottom~—sometimes even elevated by the water -—must gradually slide and descend along the declivity of the valleys or mountain slopes (r:r0up('s) wl1ich they cover. It is this slow but continual sliding of the icy masses (des gluces) on their inclined bases which carries them down into the lower valleys, and which replenishes continually the stock of ice in valleys warm enough to produce large trees and rich ha.rvests.” Very suﬂicient objections have been urged against this theory. It is evident that De Saussure considered a glacier as an accumulation of icy fragments, instead of a great a11d con- tinuous mass, throughout which the ﬁssures and “ crevasses” bear a small proportion to the solid portion ; and that he has attributed to the subglacial water a kind and amount of action for which there exists no snﬂicient or even probable evidence. The n1ain objection, however, is this, that a sliding motion of the kind supposed, if it co1n1ne11ce, 111ust be accelerated by gravity, and the glacier must slide from its bed in an avalanche. The small slope of 1nost glacier- valleys, and the extreme irregularity of their bounding walls, are also great objections to this hypothesis. The dilatation theory ingeniously meets the difliculty of the want of a sutiicient moving power to drag or shove a glacier over its bed, by calling in the well—known force with which water expands on its conversion into ice. The glacier being traversed by innumerable capillary ﬁssures, and being in summer saturated with water in all its parts, it was natural to invoke the freezing action of the night to convert this water into ice, and by the amount of its expansion to urge the glacier onwards in the direction of i.ts greatest slope. In answer to this, it is sufficient to observe, in the first place, that during the height of summer the portions of those glaciers which move fastest are never reduced below the freezing point, and that, even in the most favourable cases of nocturnal radiation producing congelation at the surface, it cannot (by well-known laws of conduction) pene- trate above a few inches into the interior of the glacier. Again, the ascertained laws of glacier—motien are (as will be immediately seen) entirely adverse to this theory, as it is always accelerated by hot weather and retarded by cold, yet does not cease even in the depths of winter. It is singular how slow observers were to perceive the importance to the solution of the problem of glacier- motion of ascertaining with geometrical precision the amount of motion of the ice, not only fro1n year to year, but fro1n day to day, in summer and winter, whether constant or variable at the same point, whether continuous or by starts; if variable, on what circumstances it depended, and in what 1na11ner it was affected at different points of the length a.nrl breadth of a glacier. This method of studying the question was taken up by Forbes. His observations were commenced on the Mer de Glace of Chamouni, in June 1842. Between the 26th a11d 27th of that month the motion of the ice opposite a point called the “Angle ” was found, by means of a theodolite, to be 166 inches in 26 hours _; between the 27th and 28th, 174 inches in 251; hours; and from about 6 -.M. to 6 P..I. on the 28th the motion was 9'5 inches, or 17'5 inches in ‘.34 hours ; whilst the proportional motion during even an hour and a half was observed. No doubt could therefore remain that the motion of the ice is continuous and toler- ably uniform-—in short, that it does not move by jerks He also ascertained about the same time that the motion of the ice is greatest towards the centre of a glacier and slower at the sides, contrary to an opinion then maintained on high authority. He next found that the rate of motion varied (529 at different points of the length of the same glacier, being on the whole greatest where the inclination of its surface is greatest. As the season advanced, he observed notable changes in the rate of motion of the same part of the ice, and connected it by a very striking direct relation with the temperature of the air. These facts were established during the summer of 1842, and promptly published. By means of occasional observations during the following winter and spring by his guide, Auguste Balmat of Chamouni, and by a 111ore full comparison of the entire motion of a glacier for twelve months with its motion during the hot season of the year, another generally received error was rectified : the motion of the glacier continues even in winter, and it has a very perceptible ratio to the summer motion. Last of all, it was found that the surface of a glacier moves faster than the ice nearer the bottom or bed. These and so1ne minor laws of motion, being undoubted expressions of the way in which glaciers move, were formu- lated by Forbes in an approximate theory: “ A glacier is an imperfect ﬂuid or a viscous body, which is urged down slopes of a certain inclination by the mutual pressure of its parts.” The analogy subsisting between the motion of a glacier and that of a river (which is a viscous ﬂuid,——were it not so, its motion would be widely different) will be best perceived by stating 1nore precisely its laws of motion. 1. Each portion of a glacier moves, not indeed with a constant velocity, but in a continuous manner, and not by sudden sub- sidences with intervals of repose. This, of course, is characteristic also of a river. 2. The ice in the middle part of the glacier moves much faster than that near the sides or banks; also the surface moves faster than the bottom. Both these facts obtain in the motion of a river in consequence of the friction of the ﬂuid on its banks, and in con- sequence also of that internal friction of the ﬂuid which constitutes its viscosity. Thus, at four stations of the Mer de Glace, distant respectively from the west shore of the glacier .... ..100 230 405 365 yds., the relative velocities were .......... .. 1 ‘O00 1 '38? 1'356 1'367. 3. The variation of velocity (as in a river) is most rapid near the sides, whilst the middle parts move nearly uniformly. This and the preceding laws are also fully brought out by the subsequent experiments of M. Agassiz on the glacier of the Aar, and of MM. Schlagintweit on the Pasterzen glacier. 4. The variation of velocity of a glacier from the sides to the middle is nearly in proportion to the absolute velocity of the glacier,—whether that absolute velocity change in the same place in consequence of change of season, or between one point and another of the length of the same glacier, depending on its declivity. See (5) and (6) below. 5. The. glacier, like a stream, has its pools and its rapids. 'here it is embayed by rocks it accumulates, its declivity in- creases, and its velocity at the same time. When it passes down a steep, issuing by a narrow outlet, its velocity increases. Thus the approximate (leclivitics of the inferior, middle, and superior regions of the Mcr dc Glace (taken in the direction of its length) are ............................................................. ..15° 4§° 8° and the relative velocities are as the numbers .. 1'398 '574 '925. 6. A fact not less important than any of the preceding is that increased temperature of the air favours the motion of the ice, a11d generally whatever tends to increase the proportion of the watery to the solid constituents of a glacier, as mild rains, and especially the thawing of the superficial snow in spring. The velocity does not, however, descend to zero even in the depth of winter. Indeed, in the lower and most accessible portions of the Mer de Glace (or Glacier des Bois) and the Glacier des Bossons, the ratio of the winter to the summer motion is almost exactly 1 : 2. On en- deavouring to establish a relation between the velocity of the glacier and the temperature of the ambient air, we find that these diminish together almost regularly down to the freezing-point, below which the velocity seems to remain constant. Any mechanical theory of glaciers 1nust be n1ore or less imperfect which does not explain the remarkable veined or ribboned structure of the ice, with its peculiar course through the interior of the glacier, as above described. According to Forbes the fundamental idea is that the veined or ribboned structure of the ice is the result of internal forces, by which one portion of ice is dragged past another in a manner so gradual as not necessarily to