EXTERNAL APPEARANCE OF GLACIERS. 297 
My failure, however, taught me to sink the 
next set of stakes ten or fifteen feet below the 
surface of the ice, instead of five ; and the ex- 
periment was attended with happier results. A 
stake planted eighteen feet deep in the ice, and 
cut on a level with the surface of the glacier, 
in the summer of 1840, was found, on my return 
in the summer of 1841, to project seven feet, 
and in the beginning of September it showed 
ten feet above the surface. Before leaving the 
glacier, in September, 1841, I planted six stakes, 
at a certain distance from each other, in a straight 
line across the upper part of the glacier, taking 
care to have the position of all the stakes deter- 
mined with reference to certain fixed points on 
the rocky walls of the valley. When I returned, 
the following year, all the stakes had advanced 
considerably, and the straight line had changed 
to a crescent, the central rods having moved 
forward much faster than those nearer the sides, 
so that not only was the advance of the glacier 
clearly demonstrated, but also the fact that its 
middle portion moved faster than its margins. 
This furnished the jirst accurate data on record 
concerning the average movement of the glacier 
during the greater part of one year. In 1842 
I caused a trigonometric survey of the whole 
glacier of the Aar to be made, and several lines 
across its whole width were staked and deter- 
13 * 
