182 KNOTT AND TANAKADATE 



the height being adjusted to suit exactly tlie leugth of the Suspension. 

 (3) The magnet chamljer — a brass tiilje with two circular glass! 

 windows, a little hirger than the magnet-mirror, and facing usually 

 north and south. Two small windows are also made facing east and 

 Avest, so that, in the process of centering, the middle of the magnet 

 (•an be seen from these directions. (4) The vice, which grips the 

 stem (ft') when the magnetometer is being carried about. This vice 

 occupies the lowest part of the magnetometer <'asing, lying insidt; 

 the part n n show^n in Figiu-e o. The details uf its c<jnstruction are 

 indicated in Figures 4, 5, G (I'late A'll). Two brass springs as as' 

 are fixed below at s's. A tube tt, with two side openings through 

 which the springs bulge out w^hen the vice is to be released, can be 

 made to slide up and down inside the magnetometer case. When 

 this perforated tube is down (Fig. 6) the vice closes ; Avhen it is up 

 (Fig. 5) the vice opens. The vice-springs grip the stem a of the 

 suspended magnet. When they are opening so as to leave the mag- 

 net free, there is considerable risk that the stem may stick to one of 

 them ; and to free it by a jerk or tap would be liable to break the 

 delicate suspension. To obviate this, two thin strips of bra^s (pp pp) 

 are fixed to the sliding tube at p'j/, and are so adjusted that their 

 upper sharpened edges press always against the inside surface of the 

 vice-springs. Thus, as the tube tt slips uj), the ends pp glide along 

 the inner faces of the springs and gently detach the stem, should it 

 chance to be sticking to either arm of the vice. The up and down 

 motion of the sliding tube is etfected by means of a special combina- 

 tion of rack and screw. Into a rectangular socket cut in the sliding 

 tube, a small piece of brass (/•, Fig. 4) tits. This piece of brass, which 

 must l)e inserted after the sliding tube has been slipped into position, 

 is toothed on its outward facing surface so as to tit into a screw cut 

 internally on the ring nn (Figs. 3 and 4.). A vertical slit, cut in the 



