M A (J N E TIC S U I! V E V OF I' E N \ s Y 1, v .\ \ I A . 



Magnetic Survey of 1840-41. 

 Recapitulation of Magnetic Results at, Girard College, Philadelphia. 

 Time of LO vibrations, reduced to temp. 60°, and correction I'm- loss of magneti 



l!» 





The daily change being known, we can compute the time of 10 vibrations at 

 Philadelphia corresponding in time to the observations made at any of the other 

 stations in 1840 and 1841, and thus obtain, by comparison, the relative horizontal 

 intensity at each station, Philadelphia being 1.000; and introducing the horizontal 

 intensity in absolute measure for Girard College, Philadelphia, we can express the 

 magnetic intensity, at all the stations visited, in the same measure. 



The secular change in the horizontal intensity has been shown l>\ Assisl 

 Schott (U. S. Coast Survey Report, 1861, Appendix, No. 22) to be small, lie 

 found, for a number of stations near the Atlantic coast, the animal secular chs 

 to be on the average — 0.001 (in parts of the horizontal force, the negative sign 

 indicating a diminution). The effect of the secular change may, therefore, be safely 

 considered as imperceptible during the interval of the magnetic survey in 1840 and 

 in 1841, each trip extending over a period of but little more than a month. 



The following table contains the duration of 10 vibrations reduced to (10° Fahr. 

 observed at stations in 1840 and in 1841, together with the corresponding duration 

 as it would have been observed at Girard College and the deduced horizontal 

 intensity, Philadelphia being 1.000. 



rn 2 



Relative horizontal intensity 77 = 



T% 



where T = time of 10 vibrations (at 60°) at Philadelphia, 

 and 7] = " » « ;i t any other station. 



