372 Introduction to the Study of Science 



Brass and glass do not interfere with the magnetic influence 

 of the earth or other magnetized body. An iron box would 

 quickly take from the needle its magnetic 

 properties. This fact as to the behavior of 

 iron is utilized in making watch cases for those 

 who frequently work about powerful mag- 

 nets, as electric generating plants. Watches 

 with iron cases are protected from the un- 

 desirable magnetic effects. 

 FIG. 133. Dial of Variation of the needle. Early in the 

 thirteenth century the magnetic needle had 

 ceased to be merely a curiosity to the educated men of Europe. 

 But little is known of its use in navigation before the age of 

 Columbus. One fact had been pointed out, namely, that the 

 needle pointed slightly west of what was supposed to be true 

 north; but this was attributed to the faulty construction of 

 the instrument. The true north was determined then, as now, 

 by observations on the sun and the north star. Columbus 

 noted this variation of the needle and by his observations 

 showed that the instrument was not wrongly constructed. 

 His observations led to an interesting discovery. 



When leaving Lisbon Columbus noticed that the needle 

 pointed west of the true north. Later when just a little west 

 of the Azores he observed to his astonishment that the variation 

 had disappeared. The needle pointed to what was determined 

 to be true north. As he continued farther westward, he found 

 the needle vary more and more toward the east of north. Later 

 observations verified his report of the facts. The magnetic 

 pole does not coincide with the geographic pole. 



The line of no declination. Columbus found that the line on 

 which the needle pointed true north or showed no declination, 

 as it is now called, a short distance west of the Azores. It is 

 now known that the line of no declination moves gradually 

 westward at the rate of about one degree in fifteen years. At 

 one time it passed through Paris ; to-day it passes along a line 



