44 A MANUAL FOR NORTHERN WOODSMEN 
that N 4° 20 ' E runs through the old spots. He figures 
now that the courses he will have to run in order to repro¬ 
duce the lines of the square are N 4° 20 ' E, S 85° 40' E, 
S 4° 20 ' W, and N 85° 40' W. He may run them so or 
turn the vernier of his compass 4° 20 ', so as to read N, E, 
S, and W, like the compass of the original surveyor. In any 
case he will not be able to reproduce the old line all around 
exactly. Even if no errors are made in either survey the 
daily variation of the needle will be pretty sure to cause 
some divergence. In remarking the line he will follow as 
closely as possible the marks of the old surveyor. 
2. By Reference Meridian. The change in bearing of 
old lines may often be ascertained by reading on a refer¬ 
ence meridian. If the compass in use be so tested and if 
the compass which did the work to be reviewed was tested 
on the same marks at the time of the original survey, then 
die difference in the two bearings will hold closely for a 
considerable region around. 
Example: On a county meridian in Pennsylvania in 
1850 a surveyor’s compass read N 2 ° 30' E and in the 
neighborhood a line was run bearing S 55° E. In 1905 
another compass on the meridian reads N 6 ° 20 ' E, show¬ 
ing a change of 3° 50' in the time elapsed. Then S 51° 10' 
E ought to reproduce the line. 
3. By Tables. The following tables, derived from 
publications of the United States Coast and Geodetic 
Survey, are very convenient for determining change in 
declination. They give for many localities well distrib¬ 
uted throughout the United States decimation at ten- 
year intervals as far back as it has been recorded. The 
change found to have taken place at a given locality 
between any two dates may then be applied through a con¬ 
siderable region around it. It should be understood, how¬ 
ever, that this means of determination does not obviate 
the chances of error due to difference between instru¬ 
ments. It is well known that two compasses on the 
same line at the same time may not read exactly alike. 
Example: A land line in the Adirondacks was run out 
in 1800 on the magnetic meridian. What course should 
be set in 1910 to reproduce it ? 
