GLACIERS. 693 



further promoted, — since glaciers move most rapidly in summer. 

 But hot, dry years would shorten it, by diminishing the ice, and espe- 

 cially at the lower end. 



Lowering the mean temperature of a place, by cooling the summers, would lower the 

 glacier-limit. Great Britain and Fuegia are in nearly the same latitude; and yet, in 

 Fuegia, the snow-line is only 3,000 feet above the sea. If, by any means, the climate 

 of Great Britain could be reduced to that of Fuegia, it would cover the Welsh and Irish 

 mountains with glaciers that would reach the sea, the snow-line being but 1,000 to 2,000 

 feet above it; and the same cause would place the snow-line in the Alps at 5,000 to 

 6,000 feet above the sea, instead of 9,000. This change of temperature involves a 

 removal of tropical sources of heat, or an increase of arctic sources of cold. 



6. The Law, Rate, and Method of Flow. — The law of flow is essen- 

 tially that of rivers. 



(1.) The movement is most rapid at or near the middle line of the 

 stream, because of friction along the sides. This is 

 proved by the observation that a straight transverse 

 line marked by poles set up in the ice {ah), becomes I • . . • 

 a curved line (c d) in consequence of the movement. 



(2.) At a bend in the stream, the movement is more rapid on the 

 convex side than on the concave ; and the medial line of greatest rap- 

 idity is nearest the convex side. 



(3.) When the stream abruptly narrows, the ice just above becomes 

 more or less heaped, and slower in movement ; and then it moves 

 through the narrows below, with a consequently increased rate of 

 flow. 



(5.) The rate of movement of the glacier as a whole depends on the 

 following conditions : — 



(a.) The amount and rate of supply of moisture precipitated as 

 snow. 



(b.) The slope of the upper surface of the glacier : which slope is 

 determined, in ordinary cases, partly by the supply 6f snow to the 

 glacier, over its upper portions, and partly by the slope and form of 

 the land beneath ; but the latter slope is not a prerequisite to move- 

 ment, as explained on page 535, just as it is not for the movement of 

 water or pitch. 



(c.) The presence or absence of obstructions, in the valley or region 

 along which it moves. 



All these points have been demonstrated by observation and experi- 

 ment. The greater rapidity of the middle portion is shown by the 

 fact that the transverse ridges made at an ice-cascade, like that of the 

 Rhone, and the lines of earth and sand in the chasms, become after- 

 ward arched in front, as shown in Fig. 1103, in which the crevasses e 

 are at first transverse, but curve below the cascade. The arch is 

 sometimes very much elongated, almost to a triangular form, as in the 



