AfAGNETIC SURVEY OF JAPAN. 189 



The vibrntion oxporiment is made in a vibration box somewhat 

 similar in construction to tlie one used in tlie Kew Instrument (See 

 Plate y\, Fi.U". 3). It is- mounted on a second tripod, so tliat tlie 

 mao-netometer stasfe need never be removed until the theodolite has 

 to be used for tiie astronomicnl observations. The maofiiet employed 

 daring- the survey was a solid steel bar of square section and polished 

 on all sides. Its length was 7-0024 cms. and its breadth '803 cms. 

 at 0° C. Its mass was Sô'OGl grammes, and its moment of inertia 

 140*15 (cnr. gr.) at 0° C. It was suspended by two loops of silk 

 from the end of a silk fibre freed from twist in the usual way by 

 means of a brass weio^ht as heavy as the masfnet. To ^et the maf»*net 

 horizontal, the base of the vibration case was first levelled bv means 

 of a spirit level. 'J'he magnet was then lowered down till it just 

 rested on the floor. Then, by raising it gently, the observer could 

 readily tell whether or not it was horizontal l)y the way in which it 

 left the floor. A few slight taps with the brush on the dij^ping end, 

 if dipping end there was, and a repetition of the same operation until 

 the mafjnet left the floor as a whole simultaneously, were sufficient to 

 make it })ractically horizontal. It was then raised to the height of 

 the side window in the box and steadied by means of a fine brush. 

 The telescope w\as adjusted and focussed till a clear image of the scale 

 was seen reflected from the polished side of the magnet. A horizontal 

 swinü" of about half a deo^ree was o-iven to the man-net l)v the aii- 

 proach and removal of a screw-driver; and the experiment conducted 

 in the usual wav. The observer sio^nalled tlie instants of transit of 

 the middle point of the swing, and these were noted down by the 

 recorder, who was posted at the other extremity of the tent with the 

 chronometer. The eye and ear method is no doubt more accurate, the 

 observer himself recording- the transits ; 1)ut to this there is the objec- 

 tion that the chronometer must be brought close up to where the 



