50 ON MAGNETISM. 



In practice there is another cause of error, from the 

 torsion-power of the suspension-thread. If, in the 

 observations above described, the thread has any 

 tendency to turn the magnet horizontally, the results 

 are certainly erroneous. To give the means of applying 

 a mechanical correction, the suspension-thread must not 

 be tied at its top to a fixed bar, but must be fastened 

 to or hung on a hook or ring attached to a small frame 

 which can be turned round horizontally. (Sometimes 

 this means of rotation is given at the lower end of the 

 thread.) And to discover whether correction is wanted, 

 the magnet must be taken out of the frame which 

 carries it, and a brass bar of equal weight and furnished 

 with a similar Reversed Telescope must be hung in its 

 place. If, on viewing with the theodolite, it is found 

 that the mark in the Reversed Telescope of the brass 

 bar occupies the same position as in that of the magnet, 

 no correction is necessary. If it does not occupy the 

 same position, the rotating frame carrying the hook 

 must be turned till the position becomes the same. 

 When this adjustment is completed, the determination 

 of the Local Magnetic Meridian will be extremely 

 accurate. 



In some instruments used for this purpose, the 

 magnet is made in the form of a hollow steel tube 

 (which can be magnetized perfectly well), and the mark 

 C and lens I) are fixed in its interior, forming a verit- 

 able telescope. 



24. Terrestrial Magnetic Meridians; Historical 

 physical changes in the system of Magnetic Meridians. 



