358 Mr. T. Gray on an Improved 



not provided for, the interpretation of the record becomes 

 exceedingly difficult ; and this difficulty is likely to be greatly 

 increased by the mass acquiring oscillations in its own free 

 period of such large angular amplitude that the direction of the 

 component which is beingrecorded becomes a variable quantity. 

 The siphon which writes the vertical component of the 

 motion is controlled by a compensated horizontal lever instru- 

 ment, on the same principle as that introduced by the present 

 writer and exhibited to the Seismological Society of Japan, 

 and described in the Transactions of that Society, vol. i. 

 part 1, p. 48, and vol. iii. p. 140, and also in the Philo- 

 sophical Magazine for September 1881. This instrument 

 consists of a horizontal lever carrying near one end a heavy 

 mass, and provided at the other end with knife-edges in a 

 line at right angles to the length of the lever. The lever 

 is supported by two flat springs, acting, through a link, on 

 a knife-edge attached to it at a point between the mass and 

 the knife-edges before mentioned, which are by this means 

 held up against the apex of inverted V-grooves rigidly 

 fixed to the framework. In the form of this instrument 

 previously described in the Philosophical Magazine, the 

 supporting springs were of the ordinary spiral type ; but in 

 subsequent instruments two flat springs have been adopted , 

 because for the same period of oscillation of the lever with- 

 out compensation they give a more compact arrangement. 

 These springs are now made of such variable breadth between 

 the fixed and the free ends that, when they are supporting the 

 lever, each part is equally bent. They may either be initially 

 straight, and bent into a circular form when in use, or they 

 may be initially set to a circular form and straight when in 

 use. When the lever is supported in this way it has a fairly 

 long period of free vibration ; and this may be increased to 

 any desired extent by means of a second pair of springs, 

 which pull downwards on a light bar fixed vertically above 

 the axis of motion of the lever. This second pair of springs, 

 besides providing the necessary compensation for the positive 

 stability of the lever and supporting-spring system, gives a 

 ready means of obtaining a fine adjustment for bringing the 

 lever to the horizontal position. This is accomplished either 

 by giving to the points of attachment of the compensating 

 springs a screw- adjustment so that they can be moved a short 

 distance backward or forward, or by making the point of 

 attachment of one spring a little in front of, and of the other 

 a little behind, the vertical plane through the knife-edge. 

 The lever can then be raised or lowered by increasing the 

 pull on one spring and diminishing that on the other. Sir 



