A. D. Bache on a Magnetic Survey of Pennsylvania. 359 
Art. XXXVII.—Abdstract of Results of a Magnetic Survey of 
Pennsylvania and parts of adjacent States in 1840 and 1841, 
with some additional results of 1843 and 1862, and a map; by 
A. D. Bacus, LL.D., F\R.S., Mem. Corr. Acad. Sci. Paris, 
Mem. Nat. A. S., Superintendent U. S. Coast Survey. 
INTRODUCTION, 
Iv the years 1840 und 1841, I made a detailed magnetic sur- 
vey 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 isomagnetic lines, the magnetic 
declination, dip and intensity; to these I added some dip and 
intensity observations in 1848, while on a tour through western 
New York and Canada. 
_ The total number of declination stations is 16, and of dip and 
“same stations 48. On assuming the duties of Superintendent 
of t S 
leisure to work up these observations, although Mr. J. Ruth and 
_ 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 re- 
_ Sults for two epochs, about 20 years apart, thus, not only giving 
_ “€ most modern values but also determining, by the known sec- 
~ ular change of the three elements, any intermediate results. 
_. The fruits of these labors, undertaken for this continent, at a 
_ omparatively early period and comprising the three elements, 
_ 4nd the whole conducted systematically, with instruments well 
Constructed for the time, will no doubt afford adequate means 2 
Watching, hereafter, the secular changes of terrestrial magnetism 
‘Within the geographical extent of this survey. 3 ae 
