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 

 the 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 W 

 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 

 decimation. They give for many localities well distrib- 

 uted throughout the United States declination 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 ? 



