(j04
EARTHQUAKES
with the caving-in of ill-supported portions of the earth’s crust, the waves from which are radiated to boundaries and then returned to their origin to coalesce and give rise to a second impulse not unlike the primary. Succeeding the first repetition of motion recorded by the seismograph there is often a rhythmical repetition of similar wave groups, suggesting the existence within our earth of phenomena akin to multiple echoes. Before considering in greater detail the nature of earthquake motion, something may be said respecting the instruments by which our knowledge of the subject has been gained. Seismo Popularly it is supposed that earthquake recorders metry. are instruments so sensitive to slight vibrations that great care is necessary in selecting a site for their installation. Although this supposition is correct for a certain class of apparatus, as for example that which will record rapid elastic vibrations produced by the movement of a train a mile distant, it is far from being so for the ordinary apparatus employed by the seismologist. What he usually aims at is either to record the more or less rapid movements of the ground which we can feel, or the slow but large disturbances which do not appeal to our unaided senses. Generally speaking, the instruments used for these purposes are not disturbed by the vibrations resulting from ordinary traffic. In almost every household something may be found which will respond to a gentle shaking of the ground. Sometimes it is a loosely-fitting shutter or window-frame, a hanging drawer-handle, or a lamp-shade which will rattle ; the timbers in a roof may creak, or a group of wine-glasses with their rims in contact may chatter. Any of these sounds may. call attention to movements which otherwise would pass unnoticed. Specially arranged contrivances which tell us that the ground lias been shaken are called seismoscopes or earthquake indicators. A small column, as for example a lead pencil standing on end, or a row of pins propped up against suitable supports, or other bodies which are easily overturned, may be used as seismoscopes. Experience, however, has shown that contrivances of this order are wanting in sensibility, and often remain standing during movements that are distinctly perceptible. A more satisfactory arrangement is one where the body to be overturned is placed upon a platform which exaggerates the movements of the ground. For example, the platform h (see Fig. 1) may be on the top of a small rod r, fixed at its lower end by plaster of Paris in a watch-glass w, and carrying a disc or sphere of lead at l. When the stand on which w rests is shaken, a multiplied representation of this movement takes place at h, and any small body resting on that point, as for example a small screw s standing on its head, may be caused to topple over. If the loaded rod is elastic its lower end may be fixed in a stand, and the spherically curved base w is no longer required. In this case the motion at h is that of elastic switching. Apparatus of this kind may be employed for several purposes beyond merely indicating that an earthquake has taken place. For example, if the falling body s is attached by a thread to the pendulum of a timepiece, it may be used to stop it and indicate the approximate time at which the tremor occurred. In its most sensitive form r is a steel wire, the upper end of which passes freely through a small hole in a metal plate. By the movement of the wire or the movement of the plate, especially if the latter projects from the top of a second and similar piece of apparatus, an electrical contact can be established hy means of which an electromagnet may ring a bell, stop a clock, or set free machinery connected with a cylinder or other surface upon which an earthquake machine may record the movement of the ground. The next class of instruments to be considered are seismometers or earthquake measurers, and seismographs, or instruments which give diagrams of earthquake motion. Although a seismograph may be designed that will not only respond to fairly rapid elastic vibrations, but will also record very slow and slight undulatory movements of the ground, experience has shown that the most
satisfactory results are obtained when special instruments are employed for special purposes. First we will consider the types of apparatus which are used to record the rapid back-and-forth movements of earthquakes which can be distinctly felt and at times are even destructive. The essential feature in these seismographs is a fairly heavy mass of metal, so suspended that although its supports are moved, some point in the mass remains practically at rest. For small earthquakes, in which the movement is rapid, the bob of a very long and heavy pendulum will practically comply with these conditions. If a style projecting from this pendulum rests upon say the smoked surface of a glass plate fixed to the ground, the vibra- O tory motion of the ground will be recorded on the glass plate as a set of superimposed vibrations. To obtain an open diagram of these movements the plate must be moved, say by clockwork. Experience, however, has shown that even when the movements of the ground are alarming the actual range of motion is so small that a satisfactory record can be obtained only by some mechanical (or optical) method of multiplication. This is usually accomplished as shown in Fig. 2. B is the bob of a pendulum, with its style s passing through a slot in the short arm of a light lever, sop, pivoted at o, and with its outer end resting upon a revolving cylinder covered with smoked paper. As shown in the figure, it is evident that the motion of o in the line sop would not be recorded, and to obtain a complete record of horizontal movements it is necessary to have two levers at right angles to each other. A complete arrangement of this kind is shown in the plan of Fig. 2. Here the style s of the pendulum rests in slots in the short arms of two writing levers pivoted at o and o'. Motion of the ground in the direction os actuates only the lever so'p', motion in the direction o's actuates only sop, whilst motion in intermediate directions actuates both. The length of the short arms of the levers is usually £ or ^ of the long arms. Although this type of apparatus is still largely employed in the Italian observatories, it has been replaced in Japan by what are called duplex pendulum seismo- up . x graphs. The change was made pen . u umS . because it frequently happened that ‘ in consequence of the movement of the ground agreeing with the period of the pendulum, the latter no longer acted as a steady point, but was caused to swing, and the record became little better than that given by a seismoscope. Very long pendulums (30 to 40 feet) are less subject to this disadvantage, but on the other hand their installation is a matter of some difficulty. A duplex pendulum (Fig. 3) consists of an ordinary pendulum diagrammatically represented by*«&, connected by a universal joint to an inverted pendulum de. The latter, which is a rod pointed at its lower end and loaded at c, would be unstable if it were not connected with b. Now imagine this system to be suddenly displaced so that a moves to a' and d moves to d'. In the new position b would tend to follow the direction of its point of support, whilst c would tend to fall in the opposite direction, and the bob of one pendulum would exercise a restraint upon the motion of the other. If, as in practice, the moment of b is made slightly greater than that of c, the system will come slowly to a vertical position beneath a'd'. In this way, by coupling together an ordinary pendulum about 3 feet in length with an inverted pendulum 2 ft. 6 in. long, it is easy to obtain the equivalent of a slowly-moving very long pendulum wffiich is too sluggish to follow" the back-and-forth movements of its supports. To complete an instrument of this description (see Fig. 4) a point in the steady mass b is used as the fulcrum for the short arm of a light-wuiting index. This has a ball joint at s, a universal joint at o, and a writing point at p, resting upon a piece of smoked glass. Attention was first directed to the possibility of rendering ordinary pendulums more truly astatic by Professor Thomas
