MMMM 



68 



zontal smfacc attaohcd to tlio earth. The ratio of lioiglit to widtli ditTors in the 

 (•ohimn.s formina; the group, and any given earthquake is expceted to throw over 

 certain of them and leave those with broader bases standing. Sand is scattered 

 round tlie base t:) prevent the overtlirown columns from rolling, and the position 

 in which they are found is exj)ccted to .-how the direction of the imjiulse (that is, 

 if the base is circular). Älallct gives a formula fa- determining the maximum 

 velocity of earth moveiuent from a knowledge of tlie height and diameter of the 

 mo.st stable column which has been overthrown. But the formula involves the 

 assumption that the motion of the base begins suddenly and continues, with uni- 

 form velocity, until the column falls; or else, that having reached its maximum 

 value witiiout disturliiug the column, the motion suddenly ceases. Either of 

 these assinn])tions, it need scarcely be said, is wholly iuai)plicable to the case of 

 an enrth<iuakc. It would be very ditiicult to predict the behaviour of a stand- 

 ing column of given dimensions, if .subjected to even the simplest of the disturb- 

 ances described in (he hi'^t chapter; and it wouhl be obviously impossible to 

 derive any dctiniti' knowledge of the cliaracter and anmunt and even the direction 

 of the motion from a knowledge of whether, and how, the column had fallen. 

 As a matter of fact, cnlumns require to be very sensitive in order to be ovcr- 

 tlu'nwn by even the sharpest shocks ordinarily experienced in Japan. Mr. 

 jNIilnc"'' has given an iiiti'rcstiug account (»f attempts to observe earthcjuakes by 

 their use. A coUnun mIiosc length was 1(1 times its diameter did not fall in any 

 earthquake during a year of Ui^v; and even when (he cnlumns were so long as to 

 rci|uire a very .«teady hand toset them up, they alhnved many earthquakes to pass 

 unrecorded. Mr. Milne adds: "Unless 1 had ])ractical experience with these 

 cohnnus it would have seemed to me incredible that the siualler of them could 

 j)Ossibly have remained standing." ITe suggests the use, in place of cylinders, 

 of columns shajK'd like an inverted bottle or a truncated cone. ITis oliservations 

 showed that columns of different sensibility sometimes fell in different directions, 

 apparently showing that the more stable had their plane of rocking changed be- 

 fore (hey fell. With sensilivc columns, especially, i( is difiicult to attain such 

 perfect .symmetry in all azimuths that a column will not fall more readily in soiue 

 directions than in others. 



Mr Milne has also tried the ])lan of })ropping up small columns (such as 

 ordinary domestic i)ins or thin stri]>s of glass) in a position as close as possible to 

 the vertical, leaving them free to fall, in an earthquake, away from the )irop. 

 The res((lts were not satisfactory. 



§ 65. MalleVs Bull Sci'smoinefcr. 



In the " Report " alreaily referred to, Mallet gives a long descriptiou of an 

 instrument intended to act as a seLsmometer, in which lialls are lield in L-shaped 

 su}ii)orts, which are fixed to the ground so as to move horizontally with it. 



* Tr.iiis. of the Seisiiiological Snciely of Jajiaii, Vol. Ill, p. 4*!, 



