26 ENGINEERING ON THE FARM 



There is an irregular line crossing the United States from 

 the east end of Lake Superior, near Lansing, Michigan, 

 Cincinnati, Ohio, and Savannah, Georgia, on which the 

 needle points north. A compass west of this line points to 

 the east of north, the variation increasing as the distance west 

 is increased. At Seattle, Washington, the needle points 

 23 ° east of true north. As a compass is taken east of this 

 line it points to west of north. At Augusta, Maine, it points 

 1 6° west of true north. When true direction is wanted, it 

 is necessary to make correction for variation, which one can 

 usually find by looking on land-survey maps, by sighting the 

 compass along a section line, or by a magnetic chart. 



Mariner's compass. The mariner's compass is one in 

 which a card or similar light material is placed on top of the 

 needle and swings with it. The directions are marked on 

 this card and thus indicate the proper direction for following 

 a given course. This make-up is most satisfactory for steer- 

 ing a vessel or for passing over lines in which the direction 

 of travel is known. 



. Surveyor's compass. The surveyor's compass is one in 

 which the letters indicating the directions are placed on a 

 ring outside the needle, which is visible. The object of a 

 surveyor's compass is to determine the direction of points 

 that are not known. The fact that the east point is on the 

 left side of the needle as it points north and the west point 

 on the right side is confusing until it is clearly understood 

 that the needle is the stationary object and the dial the mov- 

 ing part. By this arrangement of the east and west points 

 the needle in the proper direction can always be read. In 

 reading the compass, point the arrow on the dial directly 

 at the object to which the desired direction is wanted. 

 Then read the degrees under the north end of the needle, 

 using the letters on either side of the north end. In correct- 

 ing for variation, when the variation is east, add the varia- 

 tion to the reading when it is northeast or southwest, and 

 subtract when it is northwest or southeast. 



