TERRESTRIAL MAGNETIC INTENSITY. 



9 



epochs are always recorded, this correction may be applied at a future time, and 

 in a more advanced state of science. 



18. V. Variations in the Needles' Magnetism. In all observations of this 

 kind, this change gives rise to the most troublesome errors. The mode of ensur- 

 ing an equable magnetic state is unknown, though an approximation may gene- 

 rally be obtained to it. Of the two needles sent to this country in 1827, by Pro- 

 fessor Hansteen, one (No. 1.) has, after some slight variations, become almost 

 stationary in its magnetism ; the other " Flat " has been continually diminishing 

 in intensity. We have seen in the last article that the earth's magnetic action, 

 varying continually and being unknown, we can only properly compare observa- 

 tions made at the same time of the year, and of the day. The progress of change 

 in the needles may be traced by the following tables.* 



Table IV. 



NEEDLE, No. I. 



Place. 



Date. 



Observer. 



Log. Time 

 300 Vibrations. 



Log. Ratio of 

 Annual Change. 



Makerstoun, 



Edinburgh, 



Paris, 



1829, Jan. 20, 3^ 



1830, Jan. 23, 3^ 

 1829, July 9, 11^ 



1832, June 2, 11^ 



1833, May 7, 5^ 

 1836, May 4, 1^ 

 1833, June 11, 1^ 

 1835, June 13, 4^ 



Dunlop. 



Forbes. 



2.90251 

 2.90248 

 2.90765 

 2.90890 

 2.90849 

 2.90915 

 2.87102 

 2.87092 



} 



} 



The differences since June 1832 are probably imputable to the 

 tion alone. 



.99997 



.00043 

 .99966 

 .00066 



.99995 

 horary varia- 



FLAT NEEDLE. 



Place. 



Date. 



Observer. 



Log. Time 

 300 Vibrations. 



Log. Ratio of 

 Annual Change. 



Makerstoun, 

 Edinburgh, 



Paris, . . 



1828, Dec. 1, 12'' 

 1830, Feb. 14, 3'^ 



1829, July 9, 10^ 



1832, June 2, 12'> 

 1883, May 7, 6^ 



1835, May 4, 2^ 



1833, June 11, 2^ 



1836, June 13, A^ 



Dunlop. 



Forbes. 



3.00582 

 3.01435 

 3.01240 

 3.02923 

 3.03607 

 3.04579 

 2.99871 

 3.00869 



.00707 



.00546 

 .00736 

 .00488 



.00500 



* The mutual action of the needles is a point of importance. Before they came into my posses- 

 sion they were kept in their separate cases, but without farther attention, being packed together in Ihe 

 VOL. XIV. PART I. B 



