Force of the Earth's Magnetism. 23 



paired, independently of guarding them against such accidents. 

 Two of the needles have the same time of vibration now as 

 when they were first employed in 1821 ; they are both simple 

 bars ; one square at the ends, and the other pointed. A third 

 increased its time of vibration about ^th part in the first two 

 years, and has remained steady since ; it is cylindrical in 

 shape, the ends not pointed. The fourth needle altered more, 

 and was a longer time before it became steady than the others ; 

 it is a bar, with square ends, but it differs from the other 

 needles in being made (for experiment) of iron, with extremi- 

 ties of hardened steel. Two such were made at the same 

 time, and both have undergone greater changes, and have 

 been longer in becoming stationary than needles of uniform 

 hardness. 



To exemplify the manner in which the results with the 

 several needles are brought in comparison with each other, 

 and in relation to a fixed term of comparison in London, I 

 take the first station at which the needles were all employed 

 after I quitted England in 1821, namely at Sierra Leone, 

 March 20, 1822, Th. 80°, Arcs 30° to 10°. 



Needle 3. Rate U9".76 — 0.41 to Th. 60° + 0.78 for arc = 

 120".13. 



Needle 4. Rate 389".3 — 2.34 '+ 2.53 = 389".5. 



Needle 5. Rate 535".6 — 4.28 + 3.47 = 534".8. 



Needle 6. Rate 456".7 — 1.46 + 2.96 = 458".2. 

 The corresponding rates in London are, needle 3, 166" ; 

 4, 540".4; 5, 739".5; 6, 631".6. If then we take as a 

 general mean term of comparison in London, to which the 

 results at all stations should be referred, a needle which, 

 freely suspended in the direction of the dip, would make 

 its time of vibration 300", the same needle would have 

 its time of horizontal vibration in London, (the dip in 1822- 



1823 being 70°)^^ = 512".97. The time of hori- 

 zontal vibration of this needle at Sierra Leone would then be, 

 according to the several needles, as follows : — 



Needle 3. 166.0* ; 120.13' :: 512.97* \x*\ whence x = 371.2 

 Needle 4. 540.4 s ; 389.5* : : 512.92* : x* ; whence x = 369.7 

 Needle 5. 739.5* : 534.8* :: 512.92* : a?*; whence x = 371.0 

 Needle 6. 631.5* ; 458.2* ; : 512.92* : x* ; whence x = 372.2 



Mean . 371.0 



