24 ON THE STRUCTURE AND MOTION OF GLACIERS 
great scale are not wanting, although from the dazzling whiteness of 
the ice they may be very easily overlooked.” We make these 
remarks with due reserve, not having yet seen the glaciers referred to. 
The explanation just given has been brought to the test of experi- 
ment. ABCD, fig. 10, is a wooden trough intended roughly to 
represent the glacier of the Rhone, the space ACEF being meant for 
Fig. 10. 
the upper basin. Between EF and GH the trough narrows and 
represents the precipitous gorge down which the ice tumbles, while 
the wide space below represents the comparatively level valley below 
the fall, which is filled with the ice, and constitutes the portion of the 
glacier seen by travellers descending from the Grimsel or the Furka 
pass. ACLM is a box with a sluice front, which can be raised so that 
the fine mud within the box shall flow regularly into the trough, as in 
the cases already described. The disposition of the trough will be 
manifest from the section, fig. 11. While the mud was in slow motion 
Fig. 12. 
eet 
So 
<a 
3 
downwards, a quantity of dark-coloured sand was sifted over the 
space, ACEF, so as to represent the débris irregularly scattered over 
the corresponding surface of the glacier; during the passage of the 
mud over the brow at EF, and down the subsequent slope, it was 
