Page:1902 Encyclopædia Britannica - Volume 27 - CHI-ELD.pdf/658

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EARTHQUAKES

outer end of the boom. In the Isle of Wight there is. a pair of pendulums arranged as in Fig. 5. The stand is 3 feet in height. Weights of 10 lb each are carried at a distance of 10 inches from the pivots of booms which have a total length of 34 inches. With these, or even with booms half the above length, actuating indices arranged as shown in Fig. 2, but multiplying the motion six or seven times, good results may be obtained. At Rocca di Papa near Rome there is a pair of horizontal pendulums with booms 8 feet 9 inches in length, 17 feet in vertical height, which carry near their outer ends weights exceeding half a hundredweight. Although such apparatus is far too cumbersome to be used by ordinary observers, it yields valuable results. An apparatus of great value in measuring slight changes in the vertical which have a bearing upon seismometrical observation is the Darwin bifilar ^ pendulum. This consists of a mirror about half an inch in diameter, which, when it is suspended " as shown in Fig. 9, rotates by tilting at right angles to the paper. By this rotation a beam of light reflected from the surface suffers displacement. It is possible to adjust the apparatus so tha-t a tilt of sec. of arc, or a change of slope of 1 inch in 1000 miles, can be detected. The introduction of these new methods into seismometry quickly revolutionized our ideas respecting the character of earthquake motion. Although an character earthquake may be strongly felt within a of earthdistance of 50 miles from its origin, and al- 9uake though the movements in the upper storeys of niot,oabuildings within the shaken area may be large, the actual range of the horizontal motion of the ground is usually less than of an inch. With such earthquakes ordinary seismographs for recording vertical motion do not show any disturbance. Wbien the movement reaches h inch it becomes dangerous, and a back-and-forth movement of an inch is usually accompanied by destructive effects. In this latter case the amplitude of the vertical record which indicates the existence of surface waves will vary between 1 and i of an inch. In the earthquake which devas1 tated Central Japan on 26th October 1891, nearly every building within the epifocal district fell, the ground was fissured, forests slipped down from mountain sides to dam up valleys, whilst the valleys themselves were permanently compressed. The horizontal movements seem to have reached 9 inches or a foot, and the surface undulations were visible to the eye. The rapidity with which the movements are performed varies throughout a disturbance. A typical earthquake usually commences with minute Period and elastic vibrations, the periods of duration. which vary between i- and 2V °f a second. These are recorded by seismographs, and are noticed by certain of the lower animals like pheasants, which before the occurrence of movement perceptible to human beings scream as if alarmed. When, an earthquake is preceded by a sound we have evidence of preliminary tremors even more rapid than those recorded by seismographs. Following these precursors there is a shock or shocks, the period of which will be marks enabling the observer to learn the time at which a disturbance has taken place. The chief function of the instrument is to 1 or 2 seconds. From this climax the movements, measure slow displacements due to distant earthquakes. For local although irregular in character, become slower and earthquakes it will move relatively to the pivoted balance weight smaller until finally they are imperceptible. The duralike an ordinary bracket seismograph, and for very rapid motion tion of a small earthquake usually varies from a few it gives seismoscopic indications of slight tremors due to the switching of the outer end of the boom, which is necessarily seconds to a minute, but large earthquakes, which are somewhat flexible. If we wish to obtain mechanical registra- accompanied by surface undulations, may be felt for 2 or 3 tion from a horizontal pendulum of the above type, we may minutes, whilst an ordinary seismograph indicates a duraminimize the effect of the friction of the writing index—say tion of from 6 to 12 minutes. A free horizontal pendulum a glass fibre touching the smoked surface of moderately smooth paper—by using a considerable weight and placing it near to the tells us that with severe earthquakes the ground comes

period or double swing of from 15 to 30 seconds. These pendulums are usually small. The swinging arm or boom is from 4 to 8 inches long horizontally, and carries at its. extremity a weight of a few ounces. A simple form, which is sometimes referred to as a conical pendulum, may be constructed with a large sewing needle carrying a galvanometer mirror, suspended by means of a silk or quartz fibre as shown in Fig. 7. To avoid the possibility of displacements due to magnetic influences, the needle may be replaced by a brass or glass rod. The.adjustment of the instrument is effected by means of screws in the bedplate, by turning which the axis o'o" may be brought into a position nearly vertical. As this position is approached the period of swing becomes greater and greater, and sensibility to slight tilting at right angles to the plane of o'o"m is increased. The movements of the apparatus, which when complete should consist of two similar pendulums in planes at right angles to each other, are recorded by means of a beam of light, which, after reflexion from the mirror or mirrors, passes through a cylindrical lens and is focussed upon a moving surface of photographic paper. The more distant this is from the pendulum the greater is the magnification of the angular movements of the mirror. With a period of 18 seconds, and the record-receiving paper at a distance of about 15 feet, a deflexion of 1 millimetre of the light spot may indicate a tilting of ^ part of a second of arc, or 1 inch in 326 miles. Although this high degree of sensibility, and even a sensibility still higher, may be required in connexion with investigations respecting changes in the vertical, it is not necessary in ordinary seismometry. A type of instrument which has sufficient sensibility, to record the various phases of unfelt earthquake motion, and which, at the suggestion of a committee of the British Association, has been adopted at many observatories throughout the world, is shown in Fig. 8. With an adjustment to give a 15-second period, a deflexion of 1 mm. at the outer end of the boom corresponds to a tilting of the bed-plate of 0"'5, or 1 inch in 6'4 miles. The record is obtained by the light from a small lamp reflected downwards by a mirror so as to pass through a slit in a small plate attached to the outer end of the boom. The short streak of light thus obtained moves with the movement of the boom over a second slit perpendicular to the first and made in the lid of a box containing clockwork driving a band of bromide paper. With this arrangement of crossed slits a spot of light impinges on the photographic surface and, when the boom is steady, gives a sharp fine line. The passage of the long hand of a watch across the end of the slit every hour cuts off the light, and gives hour