16 EARTHQUAKES. 



some old iron nails and keys which had been attached to 

 it. From this occurrence the owner thought that the 

 magnet had, in consequence of its age, lost its powers. 

 About two hours afterwards, hoAvever, the great earth- 

 quake took place, after which the magnet was observed to 

 have regained its powers. This occurrence led to the 

 construction of the seismoscope, which is illustrated in a 

 book called the ' Ansei-Kembun-Eoku,' or a description 

 of the earthquake of 1855, and examples of the instru- 

 ment are still to be seen in Tokio. These instruments 

 consist of a piece of magnetic iron ore, which holds up a 

 piece of iron like a nail. This nail is connected, by means 

 of a string, with a train of clockwork communicating with 

 an alarum. If the nail falls a catch is released and the 

 clockwork set in motion, and warning given by the ring- 

 ing of a bell. It does not appear that this instrument 

 has ever acted with success. 



Columns, — One of the commonest forms of seismo- 

 scope, and one which has been very widely used, consists 

 of a round column of wood, metal, or other suitable 

 material, placed, with its axis vertical, on a level plane, 

 and surrounded by some soft material such as loose sand 

 to prevent it rolling should it be overturned. The fall 

 of such a column indicates that a shaking or shock has 

 taken place. Attempts have been made by using a 

 number of columns of different sizes to make these in- 

 dications seismometric, but they seldom give reliable 

 information either as to intensity or direction of shock. 

 The indications as to intensity are vitiated by the fact 

 that a long-continued gentle shaking may overturn a 

 column which would stand a very considerable sudden 

 shock, while the directions in which a number of columns 

 fall seldom agree owing to the rotational motion imparted 

 to them by the shaking. Besides, the direction of motion 



