198 The Variation of the Needle. 



In other localities the time assumed has been the year 1795, and in the 

 eastern part of Northern New York the year 1800 and 1801 are sup- 

 posed to more nearly give the time of its occurrence. 



The examination of a great number of the ancient boundary lines of 

 marked trees in our northern forests has however shown me that 

 almost all of these estimates are at fault, and that it is very rare 

 indeed the magnetic needle has ever in any locality pointed along the 

 true meridian at the dates above mentioned. 



On establishing the true meridian, at any of these localities, and 

 turning off the anciently recorded deflection angle as the direction of 

 magnetic north ; the ancient magnetic meridians prove, by this indis- 

 putable evidence, to be usually deflected far to the westward from true 

 north at localities where the lines were run at or about the years 1795, 

 1800 and 1801. Although at times easterly declinations are met with 

 — attributable to local attraction — yet the most rare condition encoun- 

 tered has been that of a close approach of any ancient or modern mag- 

 netic meridian to the true north and south line. The closest approxi- 

 mation that I have recently found has been in the county of St. Law- 

 rence and township of Granshue. Here a line, run as a magnetic 

 meridian in the year 1800, was retraced, and differed but one degree 

 and sixteen minutes from the true meridian. This single exception 

 only shows how rare has been the occurrence of the coincidence of the 

 magnetic and true meridians in this section of our country. 



Time fails me to give you in detail all the observations which I have 

 taken in Northern New York on the newly determined meridian and 

 azimuth lines of the Adirondack survey, and I believe that you will be 

 better able to grasp the facts by an inspection of the maps and diagrams, 

 on which the results of some of these observations have been plotted, 

 than by a recital of the bare numerical results. 



Theoketical Considerations. 



When we come to the consideration of the abstract scientific result 

 of such investigations, and desire to understand the origin of the power 

 which causes the declination of the needle, and the source of all the 

 variations which have been alluded to, we reach one of the most in- 

 teresting questions in physics. 



The result of a close examination will show that the apparently 

 different forces are but conditions of one and the same force; and 

 that the evidences of correlation are but the proofs of the manner in 

 which this elementary force controls the particles of matter, and by 



