MAGNETIC SURVEY OF JAPAN". 181 



The magnetometer case (Plate VI, lig. 2, and [Mate Vii. fig. 3). 

 when set in position on the central ring socket of the stage was 

 centred by means of four adjusting screws. The magnetometer is 

 shown dissected on Plate VII. Figures 1 and 2 show the magnet 

 and mirror suspension. The magnet (Fig. 2. ni) is a small hollow 

 cylinder piercing the mirror centrally pei'pendicular lo ir^ plane. 

 Mirror and magnet are fastened to an aluminium stem (««'), whose 

 lower end is broadened, no that it mav when ner-essarv be securely 

 gripped by the vice s s' shown (magnified) in figures .5 and G (Plate 

 VII). 



The suspension was by means of a spider line. .Several ex- 

 periments were made tötest the torsional effect of such a suspension. 

 A full account of these was given in the Rigaku Kyö Kwai Zasshi 

 (\'ol. 11, p. 108); but it will suffice to say here that the torsion due 

 to a twist of 180° on a spider line of the length indicated cannot 

 cause an error of 1" of ar<- in the orientation of a suspended magnet. 

 The spider-line was drawn «nu (Urectly from the living animal, and a 

 suitable length attached to the mirror and the disk d (Figs. 1 and 2), 

 from which the magnet and mirror were to l)e suspended.* The disk 

 d is a fan-shaped h(jrn damper, whose weight is nearly the same as 

 that of the mirroi* and magnet, and whose purpose, as such, is to 

 remove all torsion out of the spider line, after the suspension has been 

 carefully mounted in the case. The case consists of four parts, as 

 follows (See Plate Vll, fig. o ). (1) The top cover — a glass tube 

 capped by a bell glass, below which is a shelf (/;) with a triangular 

 hole and a slit wide enough to let the horn damper pass through 

 easily. (2) The glass tube, on which the top cover is slipped, and 

 to which it is fixed by a clamp (c) at any height within certain limits, 



* There cannot be the least doubt that the spider-line is the perfect mode of suspension of 

 a small light mirror, such as is used in delicate galvanometers and magnetometers. The 

 zero point never changes, so far at least as the suspension is concemtKl. [C. G. K.] 



