COMPASS SUE VE YING. 399 



PHYSICS AND ENGINEERING. 



COMPASS SURVEYING. 



BY FRANCIS E. NIPHER, PROFESSOR OF PHYSICS IN WASHINGTON UNIVERSITY, 



ST. LOUIS, MO. 



Those who have had to do with the survey of land by means of the ordinary 

 compass, do not need to be told that the field work is often attended with great 

 difficulties. One of these difficulties arises from the daily swing of the needle. 

 In the morning at about half past seven o'clock, the north end of the needle is 

 farthest to the east . It then moves to the west, reaching its greatest western 

 elongation at about one or two o'clock P. M. The average daily change in the 

 position of the needle between these hours is fourteen minutes, or practically a 

 quarter of a degree. This angle of swing varies, however, between eight and 

 twenty minutes, and one can never predict what the daily swing of the needle will 

 be. Hence, if a section line should be run in the morning, and the surveyor 

 should attempt to retrace the same line in the afternoon, he might find the final 

 ends of the two lines twenty feet apart. 



In addition to the daily change of the needle, it is also known that the aver- 

 age position of the needle is changing each year. The annual change at St. Louis 

 is now about two minutes a year, but the annual change is itself subject to change 

 according to a law which is as yet unknown. It is assumed that the annual 

 change is the same over large areas of the country, but it is not known that this 

 assumption is true. On the contrary, observations made at St. Louis and near 

 Washington, in Franklin county, during the last fifty years, lead to different 

 values for the annual change. In addition to these changes, which are due to a 

 ■change in the magnetic condition of the earth, there are other errors which are 

 due to a change in the magnetic condition of the needle. If the true variation 

 of the needle be known at any moment, and two compasses be set for that varia- 

 tion, and with the needles set on the zero of the circles, the sights of the com- 

 pass would not point exactly in the same direction, nor would either of them 

 point north and south, except by merest accident. If either compass should at 

 some later date be again set down at the same point, adjusted again to the true 

 variation, it would not, in general, point in the same direction as before. These 

 differences are caused by a change in the position of the magnetic axis of the 

 needle. The magnetic axis of the needle always points in the direction of the 

 magnetic meridian (when the needle comes to rest under the influence of the 

 earth's force alone), but the magnetic axis does not in general coincide with the 

 geometrical axis of the magnet, and its position is changed by the shocks and 

 jostling incident to transportation and use. These changes in the position of the 



