15° 



NA TURE 



[June 12, 1884 



tions of Japanese earthquakes do not bear this out. They 

 show, on the contrary, during the passage of almost every 

 earthquake, scores of successive movements, of which no 

 single one is very prominently greater than the rest. 

 Moreover, the direction in which a particle vibrates is so 

 far from constant that it is usually impossible to specify 

 even roughly any particular direction as that of principal 

 movement. For these reasons attempts are futile to 

 obtain knowledge of earthquake motions from instruments 

 intended to show only the greatest displacement or " the 

 direction of the shock." The indications of such instru- 

 ments are, in fact, unintelligible, and it is safe to say that 

 no seismometer is of value which does not exhibit con- 

 tinuously the displacement of a point from its original 

 position during the whole course of the disturbance. The 

 value of the observation is enormously increased if, in 

 addition to the amount and direction of the successive dis- 

 placements being shown, these are recorded in their relation 

 to the time. We can then, besides seeing the frequency 

 of the vibrations, calculate the greatest velocity of the 

 motion of the surface, and also its greatest rate of ac- 

 celeration — an element of chief importance in determining 

 an earthquake's capacity for mischief, since in a rigid 

 and rigidly founded structure the shearing force through 

 the base is equal to the product of the acceleration into 

 the mass, and the moment tending to cause overthrow is 

 that product into the height of the centre of gravity. 1 



Seismographs used during the last three or four years 

 by the writer and others in Japan give a record of the 

 earth's motion during disturbance by dividing that into 

 three components, along the vertical and two horizontal 

 lines. In the writer's apparatus these three are inde- 

 pendently recorded on a revolving sheet of smoked glass, 

 which is either maintained in uniform rotation, ready for 

 an earthquake to begin at any moment, or is started into 

 rotation (by help of an electro-magnetic arrangement) by 

 the earliest trc mors of the earthquake itself. The relative 

 position of the marks on the glass serves to connect the 

 three 1 omponents with each other, and a knowledge of its 

 speed of rotation connects them with the time. It is suffi- 

 cient that the " steady-point " for each of the three com- 

 ponents should be steady with respect to motion in one 

 direction only. It may move with the earth in either 

 or both of the other two directions, and in fact it is 

 generally most convenient to provide three distinct 

 steady-points, each with no more than one degree of 

 freedom. 



In that case each steady-point is obtained by pivoting 

 a piece about an axis fixed to the earth, and in nearly 

 neutral equilibrium with respect to displacements about 

 the axis of support. When the earth's surface shakes in 



1 The case is different a- d mt ch Ie?s simple whi re he structure is so 

 flexible as tc have a perioj of free vibration compEr.il e \i th th: periods of 

 the earthquake vibrations. 



the direction in which the piece is free to move, the sup- 

 port, which is rigid, moves with it, but the centre of per- 

 cussion of the pivoted piece remains approximately at 

 rest, and so affords a point of reference with respect to 

 which the earth's movements may be recorded. If we 

 could get rid of friction, and if it were practicable to have 

 the equilibrium of the pivoted piece absolutely neutral, 

 the centre of percussion would remain (for small motions) 

 rigorously at rest even during a prolonged disturbance. 

 But there must be some friction at the axis of support and 

 also at the tracer which records the relative position of 

 a point moving with the earth and the steady-point of the 

 eismograph. And the pivoted mass must have some 

 small stability, to prevent a tendency to creep away from 

 its normal position during a long continued shaking, or in 

 consequence of changes of the vertical. If, however, the 

 mass be so nearly astatic that its free period of oscillation 

 is much longer than the longest period of the earthquake 

 waves, and if great care be taken to avoid friction, the 

 centre of percussion behaves almost exactly as a true 

 steady-point with respect to all the most important 

 motions of even a very insignificant earthquake. The 

 effective inertia of the system may be further increased 

 by pivoting a second mass on an axis passing through the 

 centre of percussion of the first piece and parallel to the 

 axis of support. An instrument designed on these lines 

 in which the pivoted pieces in neutral equilibrium were 



vMvaaaa^, 



Fig. 3 



two light frames supported as horizontal pendulums at 

 right angles to each other, and with a massive bob pivoted 

 at the centre of percussion of each, gave (in 1 880) the 

 earliest complete records of the horizontal movement of 

 the ground during an earthquake. A description of it 

 has been given in the Proceedings of the Royal Society, 

 No. 210. 



Figs. 1 and 2 show this seismograph, improved in many 

 of its details. The form shown is one which has done 

 excellent service in a seismological observatory which the 

 writer was enabled to establish in the University of Tokio, 

 through the interest of the Japanese directors. A similar 

 instrument has also been supplied to the Government of 

 Manila. Fig. 1 shows one of the two horizontal pendu- 

 lums with a portion of one of its upright supports 

 removed. The axis of support (which slopes very slightly 

 forward to give a small degree of stability) is formed by 

 two steel points, b and c, working in an agate V-groove 

 and a conical hole. The frame of the pendulum is a light 

 steel triangle, a, the effective inertia being given almost 

 wholly by a second mass pivoted at d on a vertical axis 

 which passes through the centre of percussion of the 

 frame. The tracer, which serves to magnify as well as to 

 record the motion, is a straw, tipped with steel, and 

 attached to the pendulum by a horizontal joint at d, which 

 allows it to accommodate itself to any inequalities in the 

 height of the glass plate on which its distant end rests. A 

 portion of its weight is borne by a spring, adjustable by a 



