26 EARTHQUAKES. 



pensating arrangement, so that the pendulum, for small 

 displacements, shall be in neutral equilibrium, and the 

 errors due to swinging shall be avoided. 



Several methods have been suggested for making the 

 bob of an ordinary pendulum astatic for small displace- 

 ments. One method proposed by Gray consists in fixing 

 in the bob of a pendulum a circular trough of liquid, the 

 curvature of this trough having a proper form. Another 

 method which was suggested, was to attach a vertical 

 spiral spring to a point in the axis of the pendulum a 

 little below the point of suspension, and to a fixed point 

 above it, so that when the pendulum is deflected it would 

 introduce a couple. 



Professor Ewing has suggested an arrangement so that 

 the bob of the pendulum shall be partly suspended by a 

 stretched spiral spring, and at the same time shall be 

 partly held up from below by a vertically placed strut, the 

 weight carried by the strut being to the weight carried by 

 the spring in the ratio of their respective lengths. As to 

 how these arrangements will act when carried into practice 

 yet remains to be seen. 



Another important class of instruments are inverted 

 pendulums. These are vertical springs made of metal 

 or wood loaded at their upper end with a heavy mass 

 of metal. An arrangement of this sort, provided at its 

 upper end with a pencil to write on a concave surface, 

 was employed in 1841 to register the earthquakes at 

 Comrie in Scotland. In Japan they were largely em- 

 ployed in series, each member of a series having a 

 different period of vibration. The object of these arrange- 

 ments was to determine which of the pendulums, with a 

 given earthquake, recorded the greatest motion, it being 

 assumed that the one which was thrown into the most 

 violent oscillation would be the one most nearly approxi- 



