166 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
The apparent motion of Q measured on the moraine is greater than the true motion 
2569 + 550 6 
in the ratio — 2569 — OI ’ 5 neai 'ly- The actual motion of Qis readily deduced as well 
as the apparent rotation of the visual line QQl. Thus during 1 675 days, the duration 
of the experiment, the apparent advance of Q referred to the moraine Q’2 was twenty- 
three feet six inches, which at the distance Ql Q2 (3119 feet) subtends an angle 
of 25' 54", or almost exactly one and a half minutes daily. 
The motion of Q during the interval of any two observations of the marks (1), (2), 
(3), &c. being thus known, the correction applicable to the apparent advancement of 
the said marks beyond the visual line is at once found by the proportion 
Q Ql : Q’s motion : : Q (1) : E, 
where E is the error of apparent position of mark (1). Thus, suppose the apparent 
motion on the moraine Q2 to be seventeen inches ; this, reduced in the ratio of 6 : 5, 
gives 14 - 2 inches for the progress of Q. If the effect on the apparent place of a mark 
ninety feet from Q were required, we should have 2569 : 14'2 : : 90 : 0‘50 inch. 
I shall first detail the observations on the total motion of the glacier at Q, during 
the period to which the experiment extends, with the corrected daily motion and a 
memorandum of the state of the intervening weather, which accounts by its excessive 
variability for the remarkable variation of the progress of the glacier*. 
Table I. 
Date. 
4 
Interval. 
Apparent 
motion 
from 
commence- 
ment. 
Corrected 
daily rate. 
Weather. 
August 9. 6 p.m. 
10. 34 p.m. 
12. 2 p.m. 
17. 2| P.M. 
20. 1 p.m. 
23. 2 p.m. 
26. Noon. 
days. 
0 
0- 90 
1- 94 
502 
2- 94 
3- 04 
2-92 
ft. inch. 
0 0 
1 5 
4 0 
11 64 
16 0 
19 10 
23 6 
inch. 
15-6 
132 
14-8 j 
150 
12-5 
11-8 
9. Fine. 
10. Some rain. 11. Some rain. 
12. Some rain. 13. Snow storm. 14. Some rain. 15. Snow storm. 16. Wet. 
17. Fine; melting snow on glacier. 18. Showery. 19. Fine; snow still on 
glacier. 20. Very fine. 21. Very fine. 
22. Rain. 
23. Showery. 24. Cold rain. 25. Fine. 
26. Fine. Glacier dry. 
The first three marks on the ice, those placed thirty, sixty, and ninety feet nearer 
the centre of the glacier than Q, were fixed on the 9th of August, the mark (4) at 120 
feet was planted on the 17th, and the mark (5) at 180 feet, on the 19th. The fol- 
lowing are the observations on the apparent motions of these points past the trans- 
versal line through Q, as well as these relative motions corrected for the real move- 
ment of the station Q, as explained in last page. To avoid an illusory appearance of 
accuracy, the results are given to the nearest twentieths of an inch, which is below 
the possible errors of observation. 
* See Travels in the Alps of Savoy, &c., p. 148. 
