INTRODUCTION. 



In the years 1840 and 1841, 1 made a detailed magnetic survej of Pennsylvania, 

 and adjacent parts of New York, Ohio, and Maryland, determining at a number of 

 stations, suitably selected with regard to the course of the iso-magnetic lines, the 

 magnetic declination, dip, and intensity; to these I added some dip and intensity 

 observations in 18-43, while on a tour through Western New York and Canada. 



The total number of declination stations is 16, and of dip and intensity stations 

 48. On assuming the duties of Superintendent of the U. S. Coast Survey, in 1843, 

 I coidd not find the necessary leisure to work up these observations, although Mr. 

 J. Ruth and Mr. G. Davidson had commenced preparing, under my direction, a 

 partial abstract confined to dip and intensity observations, and to relative results. 

 In the spring of 1862, I availed myself of the services of Charles A. Schott, assist- 

 ant in the U. S. Coast Survey, who reduced, under my direction, the observations, 

 discussed the distribution of the three magnetic elements, presenting the latter 

 results also graphically, and prepared this report for the press. 



In the summer of 1862, Mr. Schott visited six of the stations previously occupied 

 by me, and redetermined the magnetic elements. Three of these stations falling 

 within the scope of the operations of the U. S. Coast Survey were at the expense 

 of the Coast Survey, the observations at the three Western stations were; secured 

 by the liberality of the Secretary of the Smithsonian Institution who, at the same 

 time, offered to publish the observations and results in the Smithsonian Contributions 

 to Knowledge. 



The observations of 1862 greatly enhance the value of my older operations, and 

 furnish the means of presenting results for two epochs about 20 years apart, thus 

 not only giving the most modern values, but also determining, by the known secu- 

 lar change of the three elements, any intermediate results. 



The fruit of these labors, undertaken for this continent, at a comparatively early 

 period, and comprising the three elements, and the whole conducted systematically, 

 with instruments well constructed for the time, will no doubt afford adequate means 

 of watching, hereafter, the secular changes of terrestrial magnetism within the 

 geographical extent of this survey. 



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