1881.] 



On a New Seismograph. 



443 



similar levers separately mounted at right angles to each other, and 

 their pointers touch the plate at different distances from the centre 

 (see Plan, fig. 2). The plate / revolves continuously in the direction 

 of the arrow about a vertical axis, which is also fixed to the earth. It 

 has not been considered necessary to show the details of this part of 

 the apparatus. 



The lever cd is proportioned so as to have this peculiarity, that the 

 axes ee and bb are axis of percussion and axis of instantaneous rotation 

 respectively. Hence, if a small horizontal displacement of the earth's 

 surface takes place in a direction at right angles to d, carrying the 

 axis ee with it, the lever will revolve through a small angle with bb as 

 its instantaneous axis (if we neglect the effect of friction). There is, 

 therefore, no tendency on the part of the bob a to acquire motion, 

 either of translation or of rotation, and it remains at rest during and 

 after the displacement. The same is true of any number of successive 

 displacements in alternate directions. 



The distance of the end of the pointer d from the axis bb is seven 

 times as great as the distance of ee from bb, and hence the pointer 

 moves through a distance seven times as great as the earth's motion. 

 But the glass plate has moved in the same direction by an amount 

 equal to the earth's motion. The recorded movement, drawn upon 

 the plate in the direction of the radius, will therefore be six times the 

 actual movement of the surface of the earth. 



' A displacement occurring at right angles to the former, or along the 

 line d, produces no record by the one lever, for pointer and plate both 

 move in the same direction and by the same amount. But this will 

 affect the second lever, which is set at right angles to the first. 



In an earthquake two rectangular components of the horizontal 

 motion are recorded by the levers to which they are respectively per- 

 pendicular, and they are magnified in the same ratio. The revolution 

 of the plate gives two continuous curves showing the successive dis- 

 placements in conjunction with the time. When a displacement 

 occurs, the pointer which has been displaced has no tendency either to 

 return or to continue moving in the same direction, until the return 

 displacement occurs, when it of course comes back. There is no 

 directive force to produce return motion; and as soon as the axis ee 

 stops moving, the lever cd comes to rest also. On account of this 

 characteristic the new instrument has been called the Astatic Hori- 

 zontal Lever Seismograph. 



The ideal action of the instrument is interfered with to a small 

 extent by friction. The friction between the marking point and the 

 plate/, and at the pivots ee, prevents the axis bb from being the true 

 instantaneous axis of the lever, and therefore there is a small amount 

 of motion of translation communicated to the bob a. Again, the 

 friction at the pivots bb tends to communicate motion of rotation to a, 



