LAYING OUT DRAINS IN THE FIELD. 8 1 



In the figure the line is AB along which the sights 

 point. The needle points constantly to the meridian, 

 hence in turning the sights to the line AB the angle 

 NB is turned off, or from o"" to 35°, and the needle 

 reads north 35° east, hence the bearing of the line is 

 N. 35° E. 



To test the accuracy of the bearing set up the instru- 

 ment at the opposite end of the line and take a back 

 sight upon the first point. If the reading agrees with 

 the first, but with opposite letters, the bearing first 

 taken was correct. The declination of the needle is the 

 angle which the magnetic meridian and the true merid- 

 ian make with each other. There is always a declina- 

 tion to take into account except on or near a certain 

 line passing across the country called the line of no va- 

 riation. The declination of the needle is constantly 

 changing. It is desirable to record lines with their true 

 bearings, or as nearly so as practicable, though this 

 feature of the work does not possess the importance 

 which is attached to it where surveys are made for the 

 definition and determination of land lines. The local 

 declination can be determined by setting up the com- 

 pass upon an old land Une whose bearing is known if 

 such can be found, or in the absence of such a line a 

 bearing may be taken upon the pole star and declina- 

 tion noted. This will be only approximate, as the star 

 is 1 1 degrees from the pole, revolving about it, and 

 being on the true meridian only twice in twenty-four 

 hours. 



Another method of determining an approximately 

 true meridian is by equal shadows of the sun. 



On the south side of a level surface, as at S in Fig. 



