SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE.

166

RECORDS AND RESULTS

OF A

MAGNETIC SURVEY OF PENNSYLVANIA

AND PARTS OF ADJACENT STATES,

IN

1840 AND 1841, WITH SOME ADDITIONAL RECORDS AND RESULTS OF 1834-35, 1843 AND 1862, AND A MAP.

BY

A. D. BACHE, LL.D., F.R.S.,

MEM. CORR. ACAD. SC. PARIS; PREST. NAT. ACAD. SCIENCES; SUPERINTENDENT U. S. COAST SURVEY.

INTRODUCTION.

In the years 1840 and 1841, I made a detailed magnetic survey 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 1843, 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 could 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, assistant 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 secular change of the three elements, any intermediate results.

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