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 EARTHQUAKES 605 Gray, who suggested methods by which this might be accomplished. ping of a pillar, may be projected, whilst the latter measures the The method shown in Fig. 4 is that devised by Professor J. A. effort exerted by an earthquake to overturn or shatter various Ewing. Records obtained from instruments of this description bodies. If after a heavy earthquake we find bodies that have give information respecting the range and principal direction of been projected or overturned, then by observing the distance motion, and show us that in a given of projection, and the height through which they have fallen, or earthquake the ground may move in their dimensions, we can by means of simple formulae calculate many azimuths. quantities closely agreeing with those obtained from the seismoFor obtaining an open diagram of gram. For example, if a body, say a coping - stone, has been an earthquake the best type of appa- thrown horizontally ratus consists of a pair of horizontal through a distance a, pendulums writing their movements and fallen from a height upon a moving surface. A simple b, the maximum horizontal velocity with Horizontal lum !'0mi aofs shorizontal penduFl 5 pendulums. consists . + of h°awnrod, ]n op, §;free > which it was projected to swing like a gate round a vertical equals lSJ ^ ; or if the or nearly vertical axis, cc', and loaded at some point b. In practice the height of the centre of of a column like weight b is pivoted on the rod whilst gravity a gravestone above the its outer end, bp, which writes on a base on which it rests smoked surface, is made extremely is y, and x is the horilight. When the frame of this zontal distance this arrangement is rapidly displaced centre from theof edge through a small horizontal range to over which it has the right and left of the direction in turned, then the accelFig. 6. which the rod points, the weight b by eration or suddenness its inertia tends to remain at rest, of motion which caused its overthrow is measured, as pointed and the motion of the frame, which is that of the earth, is magnified in the out by Mr C. D. West, with fair accuracy by the quantity gX. ratio op to bp. This apparatus, of To measure vertical motion, which with the greater number of which there are many types, was first introduced into seismometry by Pro- earthquakes is not appreciable, a fairly steady mass to which a multiplying light-writing index can be attached is fessor Ewing. Gray’s To obtain a complete record of obtained from a weight carried on a lever held by any horizontal motion, two of these pendu- form of spring in a horizontal position. Such an seismo= lums are placed at right angles; and by cranking one of the arrangement, for which seismologists are indebted to graphwriting levers, o’p’, as shown in the plan of Fig. 5, two Professor T. Gray, is shown in Fig. 6, in which B is the mass, rectangular components of the earth’s movements are written used as the steady point. This, when supported as shown, can side by side. Since the movements of the ground are frequently be arranged to have an extremely slow period of vertical motion, accompanied by a slight tilting, which would cause b or V to and in this respect be equivalent to a weight attached to a very swing or wander away from its normal position, a sufficient long spring, an alternative which is, however, impracticable. The stability is given to the weights by inclining the axis of the value of these records, as is the case with other forms of seismoinstrument at cc' slightly forwards. Although by compounding graphs, is impaired by pronounced tiltings of the ground. We next turn to types of instruments employed to record corresponding portions of the diagrams given by instruments of this type, it is possible to determine the range and direction earthquakes which have radiated from their origins, where they of the movement of which they are the resolved parts, their may have been violent, to such distances that their movements are no longer perceptible. In these instru- Instruments the same principles are followed as in the con- ments to disstruction of horizontal pendulums, the chief difference record tant earthbeing that the so-called steady mass is arranged to have quakes. a much longer period than that required when recording perceptible earthquakes. Instruments largely employed for this purpose in Italy are ordinary pendulum seismographs as in Fig. 2. The largest of these is one at Catania. It consists of a weight of 300 kilos suspended by a wire 25 metres in length, the movements of which by means of writing indexes are multiplied 12-5 times. With pendulums of shorter length, say 2 metres, it is necessary to have a multiplication 80 to 100 fold by ^ a double system of ^ very light levers, in order to render the extremely Fig. 7. Fig. 5. slight tilting of their support perceptible. This arrangement, as devised by Pro chief value is that they enable us to measure with ease the fessor Vicentini, will yield excellent diagrams of the gentle unduextent of any vibration, half of which is called its amplitude, and lations of earthquakes which have originated at great distances, the time taken to make any complete back-and-forth movement, but for local disturbances, even if the bob of the pendulum acts as a or its period. Now if a be the amplitude expressed in millimetres, point, the displacements are usually too great to be recorded and t the period expressed in seconds, then the maximum velocity steady record-receiving surfaces when so largely multiplied. of an earth particle as it vibrates to 2and2 fro equals 27ra/f, whilst onInordinary Japan, Germany, Austria, England, and Russia horizontal the maximum acceleration equals 47r a/<. The former quantity pendulums of the von Rebeur-Paschwitz type are employed, which determines the distance to which a body, as for example the cap- by means of levelling screws are usually adjusted to have a natural