298 EXTERNAL APPEARANCE OF GLACIERS. 
mined with reference to the sides of the valley ; * 
for a number of successive years the survey 
was repeated, and furnished the numerous data 
concerning the motion of the glacier which I 
have published. I shall probably never have an 
opportunity of repeating these experiments, and 
examining anew the condition of the glacier of 
the Aar ; but, as all the measurements were taken 
with reference to certain fixed points recorded 
upon the map mentioned in the note, it would 
be easy to renew them over the same locality, 
and to make a direct comparison with my first 
results after an interval of a quarter of a century. 
Such a comparison would be very valuable to 
science, as showing any change in the condition 
of the glacier, its rate of motion, etc., since the 
time my survey was made. 
These observations not only determined the 
fact of the motion of the glacier itself, as w T ell 
as the inequality of its motion in different parts, 
but explained also a variety of phenomena in- 
directly connected with it. Among these were 
the position and direction of the crevasses, those 
gaping fissures of unknown depths, sometimes 
* All the trigonometrical measurements connected with my 
experiments were very ably conducted by Mr. Wild, now Pro- 
fessor at the Federal Polytechnic School in Zurich ; they are 
recorded in the topographical survey and map of the glacier 
of the Aar, accompanying my “ Systhme Glaciaire.” 
