170 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
a series of holes from two feet to two feet, forty-five in number,, with a common car- 
penter’s centre-bit, and as nearly as possible in the visual line. The holes, which were 
^th of an inch in diameter and about five inches deep, were immediately occupied by 
wooden pins prepared for the purpose. These pins were placed as nearly as possible 
in the visual straight line, but from the nature of the operation some errors were in- 
evitable. The amount of these errors of position or zero of the marks was immedi- 
ately determined by causing the vertical wire again to traverse the series, the assistant 
placing over the centre of the head of each pin in succession the zero point of a scale 
of inches divided both ways, and held parallel to the length of the glacier, so that (the 
divisions to tenths of an inch being very plainly marked, and divisible by estimation 
by the telescope) the fundamental position of each pin was determined, and considered 
as -f- if in advance of the transverse line (in the direction of the glacier’s motion), 
and — if behind it (or nearer the origin of the glacier). The mere error of reading 
did not in any case exceed ^th of an inch, though the uncertainty of centring of 
the theodolite over Q might amount to yjyth of an inch, or even more. The two 
marks nearest Q had their positions determined by a thread stretched from the sta- 
tion-pointer of the theodolite to the third mark, their distance being too small to be 
distinctly seen by the telescope. 
The very same process, as regards the placing the zero of the scale on the head of 
the pin and reading off, was repeated on subsequent days, and the new readings minus 
the fundamental readings gave the apparent relative motion in the interval. This 
apparent motion had to be corrected, exactly as before explained, for the rotation of 
the visual line due to the translation of the fundamental point Q. 
The following Table contains — (1.) the original readings on the four days of ex- 
periment, namely — 
1844. August 20. 10 a.m. 
August 21. 6 p.m. 
August 23. 1 p.m. 
August 26. 1 1 a.m. 
(2.) The differences from the fundamental readings or total apparent displacements 
for each day, reckoning from the commencement. (3.) The same corrected for the 
rotation of the visual line from the following data : — 
August. 
Interval. 
Motion of Q. 
Correction at dist. 90 feet. 
days. 
inches. 
inches. 
20 to 21. 
1-33 
16 
0*56 
20 to 23. 
3-12 
38 
1-33 
20 to 26. 
6’08 
73 
256 
