GLACIERS. 



49 



triangle a e d. At x, therefore, where the ordinates of the triangle and 

 of the parallelogram are equal to each other, will be the lower limit of 

 the glacier. During a succession of cool years 

 the rate of melting will be represented by the 

 ordinates of the smaller triangle a e' d, and the 

 point of the glacier will advance to z. During 

 a succession of warm, dry years, the rate of 

 melting will be represented by the larger trian- 

 gle a e" cl, and the point of the glacier will re- 

 cede to y. Taking next the melting as con- 

 stant in time, and represented as before by the 

 line a e, and the motion as variable ; then, if 

 the rate of motion be represented by ordi- 

 nates of the line b c, the point of the glacier 

 will be at x as before. But, during a cycle of 

 glacial flood, the rate of motion is increased and 

 represented by a broken line b" c", and the 

 point of equilibrium is advanced to z' ; and, 

 during a cycle of diminished snow-fall and 

 shrunken glacier, the rate of motion is repre- 

 sented by V c', and the point of equilibrium 

 retreats to y' . 



Of these two factors of advance and retreat, 

 the second is probably the greatest; for, in 

 the same region and under the same climatic 

 conditions, some glaciers may be advancing and 



some retreating. The reason is as follows : As in small streams the 

 floods quickly follow the rain, while in long rivers like the Mississippi 

 the flood at the mouth may be delayed a week or ten days ; so in short 

 glaciers the ice-flood may reach the point in five or ten years, while in 

 long glaciers it may take fifty or more years.* 



Line of the Lower Limit of Glaciers. — We have said, again, that the 

 glacier reaches below the snow-line. There are three lines, or rather 

 spheroidal surfaces, running above the surface of the earth, which are 

 apt to be confounded with one another, and must, therefore, be now 

 defined. These are the line of perpetual snow, the mean line of 32°, 

 and the line of the loiuer limit of glaciers. The line of perpetual snow, 

 at the equator, is about 16,000 to 17,000 feet above the sea-level. As 

 we approach the poles it gradually approaches the sea-level, until it 

 touches at or near the poles, forming thus a spheroid more oblate than 

 the earth itself (Fig. 39). Next follows the mean line of 32°. This 

 commences at the equator, E, coincident with the snow-line (it may 



Fig. 38.— Diagram showing the 

 Causes of Advance and Be- 

 treat. The dotted lines rep- 

 resent increase and decrease 

 of melting, the broken lines 

 increase and decrease of mo- 

 tion. 



* Forel, Archives des Sciences, vol. vi, pp. 5, and 448, 1881, 



