258 ON GLACIERS IN GENERAL. 



hydrostatic pressure, derived from the ice at a higher level in 

 the rear of the point in question. Each particle is ready to 

 move in the direction in which the effective pressure is greatest. 

 Near the bottom, the frontal resistance arising from the lower 

 ice in front and itself retarded by friction, is very great, but 

 so also is the pressure of the superincumbent ice. The motion 

 of the particle will take place under the joint action of these 

 two resisting forces. As we approach the surface, the latter 

 of the two resistances (the weight of the glacier ice) is always 

 diminishing, and bears a less and less proportion to the former. 

 Near the surface, therefore, the tendency to slide will be more 

 and more directly vertical.* This consideration seems adequate 

 to explain the remarkable phenomenon of the frontal dip, with 

 its gradual fall as we approach the extremity of the glacier, 

 where, of course, the horizontal resistance from the ice in advance 

 becomes nothing. 



It has been deduced from M. Agassiz' observations on the 

 glacier of the Aar (which is remarkable for the uniformity of 

 its section, and its uniform but small slope), that the ice does 

 actually undergo a compression from back to front as it forces 

 its way down the valley ; and as ice is not sensibly compressible, 

 this diminution of the horizontal area, which any given section 

 of the glacier (between two vertical transverse planes) exhibits 

 in successive years, can only be explained by admitting that 

 the ice accumulates in a vertical direction.! 



This fact also corresponds with the convex surface which 

 the slowly-moving glaciers present. Such a surface is seen in 

 the precisely analogous case of viscous bodies, such as pitch, 

 and in clayey land-slips. 



It also satisfactorily accounts for the otherwise mysterious 



* See Seventh Letter on Glaciers. Edinburgh Philosophical Journal, 1844. 

 [Reprinted in this volume, p. 56, etc., with figures which it has not been thought 

 necessary here to repeat. See especially Fig. 15, page 59.] 



f Ninth Letter on Glaciers. Edinburgh Philosophical Journal, 1845. ("Re- 

 printed in this volume.] 



