Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/633

611 E A E T H Q U A K E Gil Fractures and fissures in walls which have been rent by an earth quake, are of great value to the seismologist, since they often indicate the direction in which the waves emerge at the surface. The interpretation of such phenomena, in some cases very compli cated, has been ably discussed by Mr Mallet, who applied the results with excellent effect to his observations on the Neapolitan earthquake. If it is possible to find, from such indications, the direction in which any two waves emerged at the surface, the depth of the seismic focus is easily determined. For since the waves radiate from this focus, any two wave-paths when produced back wards will meet at the seismic centre. It has already been shown how easily the vertical is found, and when this is known the deter mination of the focus is simplified, for as the vertical itself repre sents one-wave path it is necessary to find only one other. Let (fig. 2) be the seismic focus, and OA the seismic vertical; if a Fig. 2. wave, OB, emerge at the surface B, at an angle 8, it is evident that AO = AB. tan 0. To find the depth of the focus, it is consequently only necessary to know the angle of emergence of a wave at a given station, and the distance of this station from the seismic vertical. As the stations A and B are comparatively near each other, the earth s sphericity may be neglected, and the surface between the two regarded as practically a horizontal plane. Where several wave-paths are known, several values of AO will be obtained, and as the seismic centre is not a point, like 0, but a cavity of determinate magnitude, the average of these different values must be taken to represent the mean, focal deptli. After the great Neapolitan earthquake of 1857, Mr Mallet, aided by the lloyal Society, spent some months in carefully examining the country which had been visited by the shock ; and in 1862 he published an elaborate report in which his observations were fully discussed. By determining the wave-paths at twenty-six stations in every azimuth around the seismic vertical, he was enabled to deduce the important fact that the mean focal depth of the earthquake was about 5| geographical miles. Similar principles have since been applied by Dr Oldham to an examination of the results of an earthquake which occurred in Cachar in India, on January 10, 1869 ; and he has found that the seismic focus there must have had a deptli of about 30 miles. This coincides very nearly with the depth which Mr Mallet believes to be the maximum at which any earthquake is likely to originate in our planet. When the centre of disturbance is seated beneath the sea, as appears to have been the case with that which produced the great earthquake of Lisbon in 1755, a water-wave is generated; but since this has less velocity than the earth-wave, it does not roll in upon the shore until after the shock has been felt on land. The height of the sea-wave depends on the depth of the water. During the Lisbon earthquake the wave at Cadiz was as much as sixty feet in height. It is this great sea-wave which, breaking upon the shore after the earthquake-shock, generally completes the work of devastation. At first the water retires from the land, but in a few moments the gigantic wave rolls in, and sweeps all before it. The earthquakes which are so frequently felt on the western coast of South America are generally terminated in this manner ; and the great tidal wave which accompanied the earthquake of May 1877, wrought dreadful havoc at Arica, Iquique, and other towns on the coast. In addition to the waves propagated through earth and sea, it commonly happens that waves are transmitted through the air and thus produce sound. These sound-waves, travelling at the rate of about 1100 feet per second, may reach the observer either simul taneously with the shock or before it or even after it. They pro bably result frora sudden fracture and dislocation of rock-masses, or from subterranean explosions. Almost every object disturbed by an earthquake may be made to yield, when properly questioned, more or less information with respect to the direction and intensity of shock. Special instruments termed seismometers have, however, been constructed for this purpose, and have assumed considerable variety of form. Perhaps the simplest seismometer is that suggested by Mr Babbage, consisting merely of a bowl of some viscid liquid like treacle. On the passage of a shock the liquid rises up one side of the vessel, leaving its mark to indicate rudely the direction and extent of motion. As a modification of this simple instrument Mr Mallet proposed the use of a common wooden tub having its inside rubbed with chalk, and half filled with coloured water. An apparatus devised by Professor Cacciatore, of Palermo, and much used in Italy, is constructed with a shallow dish having eight notches in the side, and containing mercury up to the level of the lips. When any oscillation occurs, the liquid is spilt into a series of cups placed under the notches; and the quantity ejected, which may be readily weighed, gives some notion of the intensity of the shock. Since the notches face the four cardinal points and bisecting rhumbs, the direction in azimuth is approximately obtained. Mr Mallet suggested a convenient form of seismometer in the shape of a system of four L-shaped glass tubes having the upper ends closed and the horizontal limbs directed to the cardinal points. The tubes are partially filled with mercury, and the horizontal component of any shock causes the mercury to move in the lower limbs ; whilst the vertical component is determined by the motion of quicksilver in a U-shaped tube. In both cases, the movement of the liquid column is registered by means of indexes. All these instruments depend for their indications on the displacement of liquids by the shock of the earthquake. But it is obvious that the oscillations of solid bodies may be equally well employed in seisrnometry. Thus a pendulum free to move in all directions will be set vibrat ing by a shock, and may be made to record the direction and extent of its vibration by means of a stile below the bob, which moves over a bed of fine sand in a properly- shaped dish. Two pendulums are sometimes used, as pro posed by Santi. One pendulum vibrates in a vertical plane directed north and south, and the other in one striking east and west, the arcs traversed in these planes being registered by means of a tracing-stile affixed to the bob. Professor J. Forbes employed an inverted pendulum, or rod fixed at its base and weighted above, carrying at its free end a pencil or tracer by which any oscillation could be recorded. A modification of the inverted pendulum was proposed by Mr Budge of Valparaiso, in which the pendulum when moved by the first shock was kept in position at the end of a semioscillation, by means of a pawl working in a ratchet on the base of the vibrating body. Such seismometers as those previously noticed require to have their indications observed after each shock. Several ingenious instruments have, however, been con structed on self-registering principles, so that however often they are disturbed, each movement leaves a permanent record. The first of these self-registering seismometers was devised by Mr Mallet, and described in 1846. Both the horizontal and the vertical element of a shock are recorded by the movement of mercury in a system of glass tubes, the tubes being placed in a galvanic circuit so arranged that contact is broken by displacement of the liquid. As long as the circuit remains complete, a pencil traces a line on ruled paper wound round a cylinder rotated by clock-work, but the motion of the mercury intercepts the current and thus breaks the line. Two forms of &quot; ball seismometer &quot; are also due to the ingenuity of the same investigator. In one of these instruments, two heavy metul balls are placed on slightly inclined planes supported by a cast-iron table, the axis of which passes through a vertical