16 INSTRUCTIONS FOR MAKING FOREST SURVEYS, ETC. 



MAGNETIC NEEDLE. 



It is unfortunate that all makers of surveying instru- 

 ments do not have a uniform method of designating the 

 north or south end of compass needles, but that the 

 surveyor must learn and remember whether the blue or 

 white, or the weighted or cross-barred end of the needle 

 is the one which points northward. Some small com- 

 passes also differ in the positions of the E. and W. ac- 

 cording to the use which is to be made of them. If 

 they are to be used as sight compasses, they should 

 have the E. on the left side of the dial. In good weather, 

 when the sun shines or where distant features of the 

 landscape are in constant view, there is little chance of 

 error by reading the wrong end of the needle, but there 

 are many conditions under which the compass alone 

 must be the guide. 



VARIATION. 



It will be seen by the map (fig. 3) that only along one 

 line in the United States, the so-called "line of no varia- 

 tion/' does the needle point due north. This line is 

 not stationary, but has a slow movement westward. 

 At all other points in the United States the north end 

 of the needle is deflected toward the "line of no varia- 

 tion." In the North Atlantic States the variation of 

 the north end of the needle is to the west, and a sur- 

 veyor at Augusta, Me., would enter in his field notes 

 "variation 16 west." At Portland, Oreg., the entry 

 would be "variation 21J east." The maximum an- 

 nual change of variation in the United States is only 

 about 5 minutes. On the Pacific coast it is only 1 

 minute. 



