On Rapid Periodic Variations of Terrestrial Magnetism. jl 



system is very near tlie line connecting tliem. The sensilnlity may 

 be then adjusted by means of a small weight w^ playing on tlie fine 

 screw projecting on the upper face of the mirror, while the inclina- 

 tion of the mirror can be regulated by means of anotlier pai]' of small 

 nuts iv». running on the horizontal screw as shown in Fig. o, B. 



For determining the sensi):)ility, the same deflecting magnetic 

 rod as used for the preceding two instruments, was placed on the 

 pedestal Ci in the same meridian as the instrument at a distance 

 of 128*5 cm. In this case the deflector was of course applied and 

 reversed in tlie vertical position. The sensibility was so adjusted 

 tJiat the reversal of the deflector produces the displacement of 

 al)out 5 cm. on the record, or 1 mm. corresponds to 0'15 /'. 



The instrument worked very satisfactorily. Once carefully 

 adjusted, it remained so constant in every respect that it miglit 

 have run many years witJiout any furtlier trouble, except for the 

 excessive humidity of the room, which caused gradual rusting of 

 the instrument, especially of the magnetic needle, also a gradual 

 deterioration of the platinized mirror, and necessitated the com- 

 plete rearrangement of the instrument. The great advantage of 

 quartz fibre is very clearly shown in this instrument compared with 

 the others, since here no sensible creeping up of the luminous image 

 after abrupt deviation is observed, as may be seen from PI. L An 

 instrument of a similar construction may be replaced with great ad- 

 vantage for Lloyd's balance, for the usual work of lower sensibility. 



d) Illuminating and Photographic Apparatus. The source of 

 light used for photographic purposes was an ordinary acetylene 

 burner with two orifices facing each other, which was fed by a 

 capacious tank placed outside the room. The l)urner was fixed in 

 the interior of the metallic case of the lamp originally attached to 

 Mascart's magnetograph, in front of which a suitable vertical slit 

 and a cylindrical lens was inserted. The lamp was hung over the 

 pedestal Pi of the antechamber, close by the window Wi, and 

 immediately above the photographic apparatus placed on Pj. 

 The drum carrying the photographic paj^er was 24 cm. in diameter 

 and 30 cm. in length, and revolved once every 3'76 hours on an 

 average, by means of a suitable clockwork, so that 1 mm. corres- 



