1 62 PHYSICAL GEOGRAPHY. 



swinging sometimes 4 or 5 , and the pointings here indicated are the 

 means of the two extremes of oscillation. 



The Construction and Use of the Magnetic Needle. From the foregoing 

 it will be easy to deduce the value of the magnetic needle in determining 

 directions, and the precautions necessary in the use of it. In the first 

 place, the greatest care must be exercised in the construction of the 

 needle and its accompaniments. The most important points of construc- 

 tion are these : 



1. The magnetic axis of the needle should coincide with a line joining 

 its extreme points, otherwise it will fail to indicate the true magnetic 

 meridian. This would be of little consequence in using a single needle, 

 but, in comparing the work of different needles, as must frequently be 

 done, it becomes important. The magnetic axis of a needle may be 

 determined by suspending it in a stirrup by an untwisted fibre of silk 

 first one side up and then the other and observing the pointings in each 

 position. This test should be applied by the maker of the needle, and 

 the magnetic axis be made to coincide with the axis of the needle. 



2. The suspension of the needle should be such as to reduce friction to a 

 minimum. Since that component of the magnetic force, which tends to 

 bring the needle to the magnetic meridian, diminishes as the sine of the 

 angle the needle makes with that meridian, it will require but little friction 

 to cause it to stop so far out of the meridian as to introduce an apprecia- 

 ble error into the results. The best compass needles are poised upon a 

 fine needle point, in an agate or other jewelled socket ; but with such a 

 needle no less care is requisite to keep it than to make it right. The 

 more delicate the point, the more liable it is to injury, and it can be kept 

 in proper condition only by raising the needle from it when the compass is 

 moved, and letting it down carefully when to be used. The arrangement 

 for raising the needle should be a screw and not a cam, as the latter is liable 

 to work loose in transportation, and allow the needle to fall upon the point. 



3. The compass-box and tripod should be free from everytJiing magnetic. 

 Not only should no iron be used in their construction, but the brass for 

 the compass-box and tripod-head should be tested to determine whether 

 it has any power of attracting the needle. In two instances known to 

 the writer, the brass of a compass-box has become so magnetic as to 

 destroy the value of the instrument. It is easy to determine whether such 



