U A G N EX I c SURA E Y P P i: N \ S Y I. V A N I \ 



53 



1836 and ':)', ■ the dip was observed bj me al London (al W estbourne Green) and 

 found to be 69° 17'.8 in June, L837. The adopted annual decrease of the dip 

 being 2'.4, we have the dip at London, in Nov. L836, 69° 1!)'.'.', and in I i b. L837, 

 69° L8'.6. The horizontal intensity at. London (Woolwich) becomes therefore in 

 Nov. L836, 3.669, and in Feb. L837, 3.670, and in June, L837, 3.672. From the 

 general table of results we have further the relative horizontal intensities at Edin- 

 burgh (Feb. L 837), at Dublin (Nov. 1836), and at London (June, 1837), 0.841, 

 0.879, and 0.939 respectively, whence the horizontal intensities, in absolute measure, 

 at these localities and times, are for Dublin 3.436, and for Edinburgh 3.287. At 

 Philadelphia, the vibrations of the bar magnet and magnet B were observed in 

 Sept. 1836, and also afterwards at the above European stations from which, in mj 

 manuscript record, the relative intensities wore deduced as follows: Philadelphia 

 1.0000, Dublin 0.8300, and Edinburgh 0.7957. Using the above absolute values 

 for Dublin and Edinburgh, Philadelphia becomes, from comparison with Dublin, 

 4.1 -40, and from comparison with Edinburgh 4.131, mean of the determinations 

 4.136. The difference in the intensity at Girard College and the house in Chestnut 

 Street we find, by comparison of Prof. Loomis' observations in 1839 (4.1 1!)), with 

 Dr. Locke's in 1841 (4.172), is 0.023, hence the magnetic intensitj at Girard 

 College in Sept. 1836, from comparison with the European stations, becomes 

 4.159; the value actually used for the survey in 1840 was 4.160, and since- the 

 effect of the secular change for this interval of four years must be small, then is 

 no reason for changing the value adopted, it being correct within the limits of 

 uncertainty of the several comparisons. 



For comparison and the effect of secular change in X, we have the following 

 collection (from Mr. Schott's report of Jan. 1!>. 1861, CJ. S. Coast Survey Report 

 of 1861):— 



At Philadelphia. 



X. 

 is:;., o Bachc & Courtcuaj ..... 1. 195 



1836.1 Baehe .... . 1.159 



1839.5 Loomis . . . . . . .4.149 



L841.5 Locki . . . 41.172 



1842.5 Locke . . . . 4.174 

 1842.8 Lefroj ... 4.176 



1843.6 Bache . . . 4.172 

 1841..". Locke .... 4.162 



1846.4 Locke ... . 4.143 



1855.7 Schotl .... t.226 



1862.6 Schotl . . 4.08J 



A i Baltimore. 



1840."! Bache . . .... 4.265 



L841.5 Locke . . . t.261 



1841.5 Locke ... . 4.238 

 1842 8 Lcfroj . . • • 1238 



1856.7 Schotl ....... 4.203 



