MEASUREMENTS 27 



Compasses are on the market in many different forms and 

 qualities. Some of them, for pocket use, are quite small. 

 In buying a new compass the purchaser should fix definitely 

 which is the north end of the needle and be sure he under- 

 stands it thoroughly while working with known directions. 

 To keep the compass in working order he should give 

 attention to the following points: (1) The needle should 

 be straight, well balanced, well magnetized, and the agate in 

 good condition. (2) The point should be sharp and in the 

 center of the box, so that the needle can swing without strik- 

 ing the sides. (3) In using a compass, keep it a sufficient 

 distance from iron objects to prevent its being affected by 

 them. One can usually determine this fact by moving 

 around the iron object and noticing whether the needle is 

 being turned toward the object. 



Bearing. There are two kinds of bearing, magnetic and 

 true. Magnetic bearing is the bearing of the line as indi- 

 cated by the magnetic needle. True bearing is the bearing 

 of the line after the magnetic bearing has been corrected for 

 variation. 



Exact determination. Exact determination of the true 

 meridian requires careful observation of either the sun or 

 the stars and laborious mathematical computations. Solar 

 instruments are now in use which can be set up in such a 

 position that they will give the north point with sufficient 

 accuracy for surveying purposes whenever the sun is 

 shining. 



SUMMARY 



Besides the measurements given in this chapter, others, 

 such as horse-power, volts, and amperes, are discussed in 

 connection with the subjects to which they particularly 

 relate. There are many units of measure that have been 

 purposely omitted because of the infrequency with which 

 they are ordinarily encountered. The farmer would have 

 little use for a knowledge of apothecary's weights or mari- 

 ner's measure. 



