TRANSACTIONS OF SECTION A. 435 
unfavourable case we might therefore have for the hourly motion of the last stake in 
the two periods 1 and 3:9 centimetres instead of 0°6 and 5:5; that is, we should 
still have to deal with an hourly motion nearly four times as great at one time as 
at another. But the whole run of the observations shows that none but systematic 
errors could account for the discrepancy, and he has endeavoured to avoid such 
systematic errors by choosing two fixed points of reference, one of which should be 
nearly in a line with the stakes and the other as nearly at right angles to it as 
possible. In the actual case the angular distance between the two first points 
was about 113°. The regular and consistent agreement between measurements of 
that distance on different days is, he thinks, a fair test of the absence of systematic 
errors. He need not here allude to the other and obvious precautions to be taken 
in the setting up and levelling of a theodolite. The stakes were not planted 
sufficiently deep into the ice, and were often found inclined out of the vertical or 
even removed from their proper place. They had been planted near the ordinary 
tourist-track across the glacier, and were apparently considered by some of them to 
be placed there for their special benefit, to be taken out and used as alpenstocks. 
Two of them disappeared in this way, but the holes in which they had stood could 
always be recovered, and as the point of the stick sighted was always its junction 
with the ice, and as the sticks were always placed vertical with a plumb-line before 
each measurement, no appreciable error is due to any displacement of the sticks. 
The author considers, however, that the best confirmation of the results of the 
measurements is to be found in the fact that, independently of the theodolite, he 
could trace that same irregularity by simply looking along the line of sticks from 
convenient positions. 
The stakes had been planted already on July 18, and he could, without taking 
any measurements, see that even from that date up to the 2Ist the eastern side of 
the glacier had advanced very little on the eastern side compared to the western, 
while on the last day the sticks seemed to be again much more in a straight line 
than they had previously been: The eastern moraine near the place where the 
eighth stick was placed showed signs of disturbance during the night of July 
23-24, which had been stormy, snow having fallen considerably below the 
Montanvert. 
It is not easy to compare the average motion of the glacier with that of previous 
observers, because the rate of motion seems to vary considerably with the place 
across which the stakes are placed. 
When Forbes measured the glaciers were advancing, and the rate of motion 
seems to have been increasing between the years 1842 and 1846 (p. 189) ; the 
motion opposite the Montanvert seems to have been at the rate of about 2 feet a 
day, or about 2°5 centimetres an hour, and was therefore about as large, probably 
a little larger, than during the night of July 24-25 of the present year, considerably 
larger, however, than during the period July 21-25. 
Professor Tyndall made a series of observations on the Mer de Glace in the year 
1858; the place chosen for the author’s measurements coincides approximately 
with his line Bs'. His measurements give 7 inches on the western side and a 
gradual increase to 26 inches on the east ; the hourly motion would therefore vary 
from about 0-7 to about 2°7 centimetres; the relative motion of east and west is not 
very far different from that observed during the last night’s observation ; the total 
motion seems a little smaller than during that night, but in the view of the alto- 
gether different result arrived at this year during the period July 21-24 no trust- 
worthy comparison can be established. Professor Tyndall has kindly informed the 
author that, according to his recollection, the Mer de Glace was already retreating 
when he made his observation. By comparing the author's measurements with 
his, it would seem that during the first period some cause prevented the regular 
motion of the eastern side of the glacier, which cause gave way on July 24, and 
during the ensuing nicht the relative motion of the eastern and western sides came 
into good agreement with that previously observed by Professor Tyndall. 
It is much to be desired that further measurements should be undertaken, in order 
1 Phil. Trans. 1859, p. 262; 
FF2 
