RESULTS AND DISCUSSION 



OBSERVATIONS FOR MAGNETIC INCLINATION, TAKEN AT GIRARD COLLEGE, 

 PHILADELPHIA, IN 1842, 1843, AND 1844. 



THE dip circle was made by Robinson, of London, in 1836, and is six inches in 

 diameter. 1 The needles used are No. 1 and No. 2, and the poles were reversed in 

 each set of observations. The ends of the needles are marked A and B. The in- 

 strument was mounted upon a marble pier, about twenty feet to the southeast of 

 the Observatory. 



The observations for dip were made once each week, and were commenced in 

 January, 1842; they terminate in July, 1844. There are some interruptions, how- 

 ever, in the series of observation, as will be noticed in looking over the results of Table 

 No. 1. This table contains an abstract of the results taken directly from the record ; 

 it has also been compared with the synopsis of the resulting dips in Volume III of 

 the Record. The time is observatory mean time counted for convenience from O h to 

 24 h . Each mean result consists of 24 separate measures, with face of instrument 

 west and east ; marked side of the needle west and east, and with polarity north 

 and south. Three readings have been taken in each position of face of the needle. 

 With but a few exceptions, needle No. 1 was employed throughout the series ; in 

 the exceptional cases, where needle No. 2, or one of the Lloyd needles No. 1 or 

 No. 3 were used, special corrections to refer their indications to the result by needle 

 No. 1 have been deduced and applied. 



1 It is the same instrument with which I made the observations at stations in Europe (Amer. Phil. Trans. Vol. 

 VII, Part I, 1840), and those in the magnetic survey of Pennsylvania, in 1840 and 1841, and at other stations 

 farther northward and eastward, in 1843. 



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