RECORD OF CHANGES OF MAGNETIC DECLINATION. 191 



of which may be conceived as the effect of a westerly 

 magnetic force acting on the red end the magnitude 

 of the horizontal directive force H and the magnitude 

 of the vertical force V. For numerical expression of 

 all the small changes of force, it is convenient to use H 

 as the unit. The changes of declination of the free 

 magnet are in the horizontal plane, and therefore the 

 axis of the barrel on which they are registered ought to 

 be horizontal: a horizontal position also, it will be 

 shewn, can be made available for register of the changes 

 of H : but for the changes of V it will usually be found 

 convenient, though not absolutely necessary, that the 

 axis of the barrel be vertical. 



84. Record of the small changes of magnetic de- 

 clination, and evaluation of their scale. 



The apparatus, as will be gathered from the last 

 article, is exceedingly simple : a fixed source of light ; a 

 concave mirror, firmly connected with the frame that 

 carries the magnet, and causing the pencil of light to 

 converge to a spot ; and the revolving barrel with hori- 

 zontal axis which receives that spot. Suppose now that 

 the distance of the concave mirror from the surface of 

 the barrel where the spot is formed is m inches. To 

 give to the spot a motion of 1 inch, the beam of reflected 



light must have been turned through the angle : and 



YH/ 



therefore, as the direction of the incident light is in- 

 variable, the mirror (and the magnet which accompanies 



