MAGNETIC SURVEY OF JAPAK. 



179 



several |)ni"po«e.s. It consist.s essentially of a theodolite, n niirvnr 

 inaunetometer. a declination roil, a small o-alvanio cell, a resi.stance 

 box, a vibration case, a detlection bar, and a bar maenet. At any 

 given station, the base of the theodolite is fixed and adjusted once for 

 all, and, if possible, never chanired throughout the whole series of 

 experiments. Only in this way can a really satisfactory series of 

 diurnal observations of the declination be made. That this might be 

 done, a second tripod was necessary for mounting the dipping circle 

 and vibration apparatus.* 



The instruments will be described (1) as a Declinometer, (2) 

 as an apparatus for measuring the horizontal force, (3) as an alt- 

 azimuth or transit instrument for determining the astronomical 

 meridian, the clock error, and latitude. 



The theodolite, which formed the basis of the whole, was one of 

 Negretti and Zambra's construction. To fit this up as a magnetometer, 

 the compass needle had first to be removed, and into the hollow space 

 left in the centre of the theodolite base, was fitted what may be called 

 a magnetometer stage. In figures 1 and 5, Plate VI., this stage may 

 be seen, and its form is specially will shown in figure 5. In the 

 subjoined cut. it is shown in clearer detail. The chief points aimed 

 at in its construction were ease of adjustment and facilities for clamping- 

 it either to the base of the theodolite or to the Y's. The disk-shaped 

 base of the stage with its circumferential ring-clamp (c) nearly fitted 

 the circular cavity left after removal of the theodolite compass needle. 

 This ring-clamp, when tightened, abutted against the sides of the 

 receptacle, and fixed the whole stage to the V"s of the theodolite. 

 When loosened, its two halves collapsed upon the disk-shaped base 

 of the stage, which fhen simply rested, undamped, upon the plat- 



* A second tripod might will be added to the Kew set of instruments, and a distinct 

 saving of time be effected in mnking a series of observations throiisjhont the day. [C. G. K] 



