Location of the Isogontc Lines. 



1*7 



ation charts, is of unquestionable value as they confirm, enforce and 



anticipated in any local work of observation: I, .it tlu- closer the stations 

 are located the more exact becomes our charting of these theoretic 

 representations of the magnetic force, and the more -wonderful, ele- 

 gant and valuable are the results both in the increased accuracy of the 

 information conveyed and the corresponding practical advantages, so 

 important in some of the special surveys which I have had recently to 

 conduct on behalf of the State. 



Such must be the explanation of the utility of these special local 

 observations of the declination of the magnetic needle from the true 

 meridian. I have only to regret that the limited means and restricted 

 objects of the work did not permit the observation of all the ele- 

 ments, into which mechanical contrivances have divided the sum of the 

 magnetic forces; yet will hope that, in future, broader views may enable 

 us to secure much scientific data without any loss to the purely prac- 

 tical and utilitarian part of our work. 



Before giving an account of the results of observation in Northern 

 New York, some mention of the methods employed will he appropriate. 



The True Meridian. 



A knowledge of the location of the true meridian is, of course, the 

 first essential element in ihe study or investigation of the closeness of 

 the indication of the magnetic needle of the direction of true north, 

 or of its declination to the eastward or westward from the direction of 

 the geographical pole. 



A new meridian line is only to be determined by astronomical obser- 

 vations ; but when one such true meridian has been established, other 

 secondary meridians may be located by triangulation to the east or 

 west, and thus the azimuths of a great primary triangulation make 

 thetae mma.au e»y of reference at all the signal statu™ of the 



