CHAPTER VI. 



THE USE OF THE MAGNETIC NEEDLE IN SURVEYING. 



By E. T. QUIMBY, a.m., 



PROFESSOR OF MATHEMATICS AND CIVIL ENGINEERING, DARTMOUTH COLLEGE. 



>HE object of this paper is to explain the facts of terrestrial mag- 

 'f% netism, so far as they relate to the use of the magnetic needle by 

 the surveyor, with particular reference to the state of New Hampshire. 

 It will not therefore be necessary to describe the construction and use of 

 the instruments by means of which these facts have been observed, nor 

 to discuss the formulae for the reduction of the observations. Those 

 who wish to make a thorough examination of this subject are referred to 

 the works of Airy, Walker, and others, and to the reports of the United 

 States Coast Survey, under whose auspices extensive magnetic observa- 

 tions have been and are still being made in various parts of our country. 

 It may seem of little importance to reproduce what has been so long 

 known, when nothing specially new can be added ; but an examination of 

 the records of surveys made within the last fifty years will show that 

 there is need either of more general knowledge on this subject, or of a 

 better use of what is known. It is quite unusual to find in any of these 

 records the slightest reference to magnetic declination; and there is 

 reason to believe that surveyors sometimes rely too implicitly upon the 

 needle in retracing old lines by their former magnetic bearings. It will 

 appear by the behavior of the needle that, while it is a valuable aid, it can 



