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SEISMOMETER (from Gr. veurp6s, earthq...
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See also:SEISMOMETER (from Gr. veurp6s, See also:earthquake, and µfpov, a measure) . This name was originally given to See also:instruments de-signed to measure the See also:movement of the ground during See also:earth-quakes (q.v.). Observations have shown that, in addition to the comparatively See also:great and sudden displacements which occur in earthquakes, the ground is subject to other movements. Some of these, which may be called " earth-tremors," resemble earth-quakes in the rapidity with which they occur, but differ from earthquakes in being imperceptible (owing to the smallness of the See also:motion) until instrumental means are used to detect them. Others, which may be called " earth-tiltings," show themselves by a slow bending and unbending of the See also:surface, so that a See also:post See also:stuck in the ground, See also:vertical to begin with, does not remain vertical, but inclines now to one See also:side and now to another, the See also:plane of the ground in which it stands shifting relatively to the See also:horizon. No See also:sharp distinction can be See also:drawn between these classes of movements. Earthquakes and earth-tremors grade into one another, and in almost every earthquake there is some tilting of the surface. The See also:term " seismometer " may conveniently be extended (and will here be understood) to See also:cover all instruments which are designed to measure movements of the ground. Popularly it is supposed that earthquake recorders are instruments so sensitive to slight vibrations that great care is necessary in selecting a site for their See also:installation. Although this sup- position Setsmois correct for a certain class of apparatus, as for Se sm example that which will See also:record rapid elastic vibrations See also:pro- duced by the movement of a See also:train a mile distant, it is far from being so for the See also: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 See also:appeal to our unaided senses. Generally speaking, the instruments used for these purposes are not disturbed by the vibrations resulting from ordinary See also:traffic. In almost every See also:household something may be found which will See also:respond to a See also:gentle shaking of the ground. Sometimes it is a loosely-fitting shutter or window-See also:frame, a See also:hanging drawer-handle, or a See also:lamp-shade which will rattle; the timbers in a roof may creak, or a See also:group of See also:wine-glasses with their rims in contact may chatter. Any of these sounds may See also:call'See also:attention to movements which otherwise would pass unnoticed. ,Specially arranged contrivances which tell us that the ground has been shaken are called seismoscopes or earthquake indicators. A small See also:column, as for example a See also:lead See also:pencil See also:standing on end, or a See also:row of pins propped up against suitable supports, or other bodies which are easily over-turned, may be used as seismoscopes. Experience, however, has ' Up to the See also:middle of the 15th See also:century "See also:seisin " was applied to chattels equally with freeholds, the word " possessed " being rarely used. In course of See also:time the words acquired their See also:modern meaning. See F. W. See also:Maitland, "Seisin of Chattels," See also:Law Quarterly See also:Review, vol. I. p. 324 and The See also:Mystery of Seisin," Law Q. R. ii. 481. See also:Pollock and Maitland, Hist. Eng. Law, vol. ii. 29 seq.; See also:Fry, L. J., in Cochrane v. See also:Moore (189o), 25 Q.B.D. 57. 590 shown that contrivances of this See also:order are wanting in sensibility, and often remain standing during movements that are distinctly perceptible. A more satisfactory arrangement is one where the See also:body to be overturned is placed upon a See also:platform which exaggerates the movements of the ground. For example, the platform h (see fig. I) may be on the See also:top of a small See also:rod r, fixed at its See also:lower end by See also:plaster of See also:Paris in a See also:watch-See also:glass w, and carrying a disk or See also:sphere of lead at 1. When the stand on which w rests is shaken, a multi-plied See also:representation of this movement takes See also:place at h, and any small body resting on that point, as for example a small See also:screw s standing on its See also: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 See also:base w is no longer required. In this See also:case the motion at h is that of elastic switching. Apparatus of this See also: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 See also: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 See also:form r is a See also:steel See also:wire, the upper end of which passes freely through a small hole in a See also:metal See also: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 See also:electrical contact can be established by means of which an electromagnet may See also:ring a See also:bell, stop a See also:clock, or set See also:free machinery connected with a See also:cylinder or other surface upon which an earthquake See also: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 Setimo- give diagrams of earthquake motion. Although a seismometer. graph may be designed that will not only respond to Selsmo- fairly rapid elastic vibrations, but will also record very :mph. slow and slight undulatory movements of the ground, experience has shown that the most satisfactory results are obtained when See also: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 See also:felt and at times are even destructive. The essential feature in these seismographs is a fairly heavy See also:mass of metal, so suspended that although its supports are moved, some point in the mass remains practically at See also:rest. For small earthquakes, in which the movement is rapid, the bob of a very See also:long and heavy pendulum will practically comply with these conditions. If a See also:style projecting from this pendulum rests upon say the smoked surface of a glass plate fixed to the ground, the vibratory motion of the ground will be recorded on the glass plate as a set of superimposed vibrations. To obtain an open See also: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 ob- tained only by some See also:mechanical (or See also:optical) method of multiplica- tion. 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 See also:short See also:arm of a See also:light See also:lever, sop, pivoted at o, and with its See also:outer end resting upon a revolving cylinder covered with smoked See also:paper. As shown in the figure, it is evident that the motion of o in the See also:line sop would not be recorded, and to obtain a See also:complete record of See also: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 See also:plan of fig. 2. Here the style s of the pendulum rests in slots in the short arms of two See also:writing levers pivoted at o and o'. Motion of the around in the direction os actuates only the lever so'p', motion In the direction o's actuates only sop, whilst motion in inter- mediate directions actuates both. The length of the short arms of the levers is usually * or II, of the long arms. This type of apparatus has been replaced in See also:Japan by what are called duplex pendulum seismographs. The See also:change was Duple: made because it frequently happened that in consequence pendu- of the movement of the ground agreeing with the See also:period lams. of the pendulum, the latter no longer acted as a steady point, but was caused to See also:swing, and the record became little better than that given by a seismoscope. Very long pendulums (30 to 40 ft.) are less subject to this disadvantage, but on the other See also:hand their installation is a See also:matter of some difficulty. A duplex pendulum (fig. 3) consists of an ordinary pendulum diagrammatically represented by ab, connected by a universal See also:joint to an inverted pendulum dc. The latter, which is a rod pointed at its lower end and loaded at c, would be unstable it it were not connected with b. Now imagine this See also: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 See also: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 ft. in length with an inverted pendulum 2 ft. 6 in. long, it is easy to obtain the See also:equivalent of a slowly-moving very long pendulum which is too Ic sluggish to follow the back-and-forth movements of its supports. FIG. 3.
To complete an See also: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-writing See also:index. This has a See also: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 See also:Professor homas See also: A See also:simple form of Rorizontal horizontal pendulum as shown in fig. 5, consists of a rod, pendu- op, free to swing like a See also:gate See also:round a vertical or nearly lams. vertical See also:axis, oo', and loaded at some point b. In practice the See also:weight b is pivoted on the rod whilst its outer end, bp, which writes on a smoked surface, is made extremely light. When the frame of this arrangement is rapidly displaced through a small horizontal range to the right and See also:left of the direction in which the rod points, the weight b by its inertia tends to remain at rest, and the motion of the frame, which is that of the earth, is magnified in the ration op to bp. This apparatus, of which there are many types, was first introduced into seismometry by Professor Ewing. To obtain a complete record of horizontal motion, two of these pendulums are placed at right angles; and by cranking one of the writing levers, o'p', as shown in the plan of fig. 5, two rectangular components of the earth's movements are written side by side. Since the movements of the ground are frequently accompanied by a slight tilting, which would cause b or b' to swing or wander away from its normal position, a sufficient stability is given to the weights by inclining the axis of the instrument slightly forwards. Although by compounding corresponding portions of the diagrams given by instruments of this type, it is possible to determine the range and direction of the movement of which they are the resolved parts, their See also:chief value is that they enable us to measure with ease the extent of any vibration, See also:half of which is called its See also:amplitude, and the time taken to make any complete back-and-forth movement, or its period. Now if a be the amplitude expressed in millimetres, and t the period expressed in seconds, then the maximum velocity of an earth particle as it vibrates to and fro equals era/t, whilst the maximum See also:acceleration equals 4r2a/12. The former quantity determines the distance to which a body, as for example the capping S 31. A of a See also:pillar, may be projected, whilst the latter See also:measures the effort which have originated at great distances, but for See also:local disturbances, exerted by an earthquake to overturn or shatter various bodies. If after a heavy earthquake we find bodies that have been projected or overturned, then by observing the distance of See also:projection, and the height through which they have fallen, or their dimensions, we can
by means of simple formulae calculate quantities closely agreeing with those obtained from the seismogram. For example, if a body, say a See also:coping-See also: Instruments record dta- largely employed for this purpose in See also:Italy are ordinary taut earth- pendulum seismographs as in fig. 2. One at See also:Catania quakes. 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 8o to See also:loo See also:fold by a See also:double system of very light levers, in order to render the extremely slight tilting of their support perceptible. This arrangement, as devised by Professor G. Vicentini of See also:Padua, will yield excellent diagrams of the gentle undulations of earthquakeseven if the bob of the pendulum acts as a steady point, the highly multiplied displacements are usually too great to be recorded. In Japan, See also:Germany, See also:Austria, See also:England and See also:Russia horizontal pendulums of the von Rebeur-Paschwitz type are employed, which by means of levelling screws are usually adjusted to have a natural period or double swing of from 15 to 30 seconds. These pendulums are usually small. The swinging arm or See also:boom is from 4 to 8 in. 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 See also:needle carrying a See also:galvanometer See also:mirror, suspended by means of a See also:silk or See also:quartz fibre as shown in fig. 7. To avoid the possibility of displacements due to magnetic influences, the needle may be replaced by a See also:brass or glass rod. The See also:adjustment of the instrument is effected by means of screws in the See also:bed-plate, 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 See also:beam of light, which, after reflection from the mirror or mirrors, passes through a cylindrical See also: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 ft., a deflection of I millimetre of the light spot may indicate a tilting of h See also:part of a second of arc, or 1 in. in 326 See also: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 very sensitive modified von Rebeur instrument was employed by O. See also:Hecker in his measurement of the variation in the vertical and of tidal earth tremors. A type of instrument which has sufficient sensibility to record the various phases of unfelt earthquake motion, and which, at the See also:suggestion of a See also:committee of the See also:British Association, has been adopted at many observatories throughout the See also:world, is shown in fig. 8. With an adjustment to give a 15-second period, a deflection of i mm. at the outer end of the boom corresponds to a tilting of the bed-plate of o"•5, or I in. in 6.4 m. 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 dumanaeOItO,punnIr!I Il ^ NWa '\\\\\\\\\\\\\\~ Boom 592 the movement of the boom over a second slit perpendicular to the first and made in the lid of a See also:box containing clockwork See also:driving a See also: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 See also:fine line. The passage of the long hand of a watch across the end of the slit every See also:hour cuts off the light, and gives hour marks enabling the observer to learn the time at which a disturbance has taken place. The chief See also:function of the instrument is to measure slow displacements due to distant earth-quakes. For local earthquakes it will move relatively to the pivoted See also:balance weight like an ordinary See also:bracket seismograph, and for very rapid motion it gives seismoscopic indications of slight tremors due to the switching of the outer end of the boom, which is necessarily somewhat flexible. If we wish to obtain mechanical See also:registration from a horizontal pendulum of the above type, we may minimize the effect of the See also:friction of the writing index—say a glass fibre touching the smoked surface of moderately smooth paper—by using a considerable weight and placing it near to the outer end of the boom. In the Isle of See also:Wight there is a pair of pendulums arranged as in fig. 5. The stand is 3 ft. in height. Weights of to lb each are carried at a distance of to in. from the pivots of booms which have a See also:total length of 34 in. 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, See also:good results may be obtained. At Rocca di Papa near See also:Rome there is a pair of horizontal pendulums with booms 8 ft. 9 in. in length, 17 ft. in vertical height, which carry near their outer ends weights exceeding half a See also:hundred-weight. 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 See also:Darwin bifilar pendulum. This consists of a mirror about half an See also:inch in See also: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 re- flected from the surface suffers displacement. It is possible to adjust the apparatus so that a tilt of Tina sec. of arc, or a change of slope of 1 in. in woo miles, can be detected. (See See also:Sir G. H. Darwin, Scientific Papers, vol. i. (1907)) The principle of the Vicentini instrument described above has been adopted by G. Agamennone, director of the See also:observatory at Rocca di Papa, near Rome, and also by E. Wiechert of See also:Gottingen. In the Agamennone seismometrograph the pendulum is See also:cheese-shaped, and weighs 500 kilos in one form and 2000 kilos, or over two tons, in the largest. This cylinder, which is suspended from a stand rigidly attached to the earth, has a vertical hole in its
:entre extending from its upper surface to its centre of gravity, and to the bottom of this well a light rod is fixed. The motion of the frame is communicated to this rod by an See also:extension of the frame which makes contact with it just above its point of See also:attachment to the well. The motion is first magnified by the lever, and, on its communication to a complex lever system above the stationary mass, is still further magnified before registration, which is effected by a See also:pen supplied with See also:ink writing on See also: Wiechert has also constructed a seismometer on the same principle, but in which the stationary mass is smaller, being adjustable between 8o and 20o kilos (18o and 440 lb).
The See also:Strassburg or See also:Bosch seismograph differs from those just de-scribed in resembling the Milne instrument, i.e. it is a horizontal and not a vertical pendulum. The steady mass, however, is much larger, being too kilos (or 220 lb) ; the magnification is from 8o to too; and the registration is effected on a See also:roll of smoked paper. An air-damping apparatus is attached in order to annul the natural oscillations of the pendulum. Two of these instruments are set up, one in the N.-S. direction and the other in the E.-W. so as to record the two horizontal components. A more popular Strassburg instrument has a stationary mass of 25 kilos. The Galitzin seismograph, devised by See also:Prince
Galitzin, is of the same type, but it essentially differs from the Milne instrument in having its pendulum dead-See also:beat; this is brought about by an electromagnetic See also:device. Magnification and registration of the motion is effected in the following way. Attached to the pendulum is a coil of fine wire which moves in the See also: Additional information and CommentsThere are no comments yet for this article.
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