Location of the Isogomc Lines 



determinations of the direction of true north must be made. These 

 meridian lines are often detached from the main system of triangula- 

 tion, and upon the surveys of the public lands under the superintend- 

 ence of the writer are located for the purpose of testing the compas- 

 ses of the local land surveyors, and ascertaining the true azimuths of 

 the lines run. For such work, over rough and difficult country, where 

 there are no roads, and rarely any paths or trails, the large and pon- 

 derous theodolites are rarely available, and always inconvenient, re- 

 quiring much night work and delay. Their weight and cumbersome- 

 ness usually prevent their employment on the township and lot lines 

 in the forest, and a desire for convenience and rapidity in work lias 

 led to the use of other instruments, light enough to be carried by one 

 of the surveyor's assistants, and so made as to be readily adjusted and 

 available for work at almost any time. 



Life is so short, and time so much an element of utility, that these 

 portable instruments, furnishing the necessary information within the 

 limits of accuracy required on township or lot lines — and, by careful use, 

 yielding valuable scientific information in regard to terrestrial mag- 

 netism at hundreds of localities from which we should otherwise have 

 no information whatever— are found to be of the greatest convenience. 



Field Methods of Observation. 

 While the time at disposal will not admit of my giving an account 

 of all the instruments that have been devised to facilitate the deter- 

 mination of the declination of the needle upon land-line surveys, it is 

 believed that a brief sketch of the growth or evolution of the instru- 

 mental forms now in use will assist the unprofessional to an under- 

 standing of the work to be done, and the way in which the results are 

 obtained. 



Disregarding the vague descriptions of the more ancient instru- 

 ments and the delicate torsion instrument, and dealing only with 

 those in practical use in the daily work of navigators and engineers, 

 the first form of declinometer that we meet with in general use is the 

 azimuth compass, a nautical implement with which the greater num- 

 ber of observations of the "variation" at sea have been taken. 



The azimuth compass of the navigator is a large instrument, hung 

 in gimbals, yet its sights are easily turned in any direction so as to aid 

 in taking magnetic bearings to any object. The true bearing or 

 azimuth of any such magnetic sight-line is usually obtained at sea by 

 the " amplitude of the sun/' that is its bearing near sunrise or sunset, 

 or the azimuth and altitude of the sun (the altitude being taken with 



