1800.] 



On a new Magnetometer. 



265 



w 



Fly.1 



of thin sheet brass, some 1'3 cm. in width, which is bent over at 

 the top to form a wide hook, S, figs. 1 and 2, under which the bifilars 

 pass. At the top of the instrument is attached a similar hook, S', 

 from which the bifilars are suspended. The widths of these hooks 

 (which . form the distances between the bifilars at the top and 

 bottom) can be read with a micrometer gauge with certainty to 

 O'OOl cm., and by estimation more accurately. The circular magnet, 

 bar, and attached mirror are made as light as they can possibly be 

 made. 



Pieces of wood (X, X) are attached to the bottom and two shallow 

 sides of the box in which the circular magnet is suspended in such a 

 way as to leave only about O'l cm. clearance for the magnet when it 

 is oscillating before coming to rest. As the needle, too, can only 

 vibrate freely when within a millimetre or so of the centre, it is clear 



VOL. XLVTII. u 



