The seismograph was afterwards set up just above the pier at the south end of the span in the middle of which it had previously been standing, and five more records were obtained in the new position. Except that the motion was somewhat less, they had much the same characteristics as before. The following notes refer to the passage of a slow goods-train from Dundee as observed from this position : Mins. Secs. 30 : 40 Train entered bridge: minute longitudinal oscilla- 33 0 36 38 10 Lateral oscillation began. Train passed seismograph. Tail van of train off bridge: oscillation ceased. In all seismometric work, whether it be the measurement of earthquakes proper, or of such shakings as these, the trustworthiness of the record depends on the degree to which the presumed "steadypoint" of the instrument remains at rest during a protracted disturbance of the base. The accuracy of a seismograph admits of easy experimental test in the manner which the author described and illustrated when communicating to the Royal Society an account of his Horizontal Pendulum Seismograph, for recording separate components of motion upon a moving plate.* The test consists in placing the instrument upon a stand which may be shaken by hand, and causing a true autograph of the motion of the stand to be drawn. by an independently supported index, side by side with the record. that is drawn by the seismograph itself. Fig. 3 shows how this test was applied to the instrument with which the Tay Bridge observations were made. The seismograph was mounted on a stand which was constructed to give it two degrees of freedom of horizontal translation, without freedom to rotate. This was done by laying a pair of turned steel rollers parallel to each other on the top of a steady level table; a small drawing-board rested on them; on the top of it a second pair of steel rollers were laid at right angles to the pair below; a second small drawing-board lay on them, and the instrument stood upon it. The upper board was then free for translation in all azimuths, and was shaken by hand so that it imitated the motion in an actual earthquake. A record of this motion was drawn by the seismograph index, and beside it a second record was drawn by the lever and index g (fig. 3) which was held by a gimbal joint in a stiff bracket h secured to the upper board, and took its motion from a true steady-point i obtained by making the bottom end of the lever in the form of a small ball socketed in a 6 * "On a new Seismograph," Roy. Soc. Proc.,' vol. 31, 1881, p. 440. cylindrical hole in the bracket j, which was firmly fixed to the (motionless) top of the table. When the multiplication given by this lever g is arranged to be the same as that given by the seismograph the two records should be identical, except for error caused by the "steady-point" of the seismograph wandering through friction, or because of the stability of the suspended mass, and except for those errors which both the seismograph and the testing lever are liable to FIG. 3.-Arrangement for testing the Duplex Pendulum Seismograph. through backlash at the joints and want of rigidity in the lever and index arm. In practice the agreement between the records is most satisfactory. Fig. 4 gives examples of the result of this test as applied to the seismograph which was used upon the Tay Bridge, when the shaking was made to imitate such movements as the ground executes in small and in large earthquakes. Tests of this kind not only demonstrate the accuracy of the seismograph, but are a convenient means of finding experimentally the ratio in which the recording index multiplies the motion of the ground. FIG. 4.-Comparison diagrams to test accuracy of Duplex Pendulum Seismograph. For an exhaustive examination of the vibration of a structure under "live" loads, the more elaborate type of seismograph might be used, which records linear components of the motion on a surfac that is moved uniformly by clockwork, The usual form of this instrument comprises two horizontal pendulums, for the two horizontal components, and a third piece which is suspended astatically with freedom to move up and down only for the vertical component.* This arrangement employs a distinct mass and a distinct "steadypoint" with respect to each component. The duplex pendulum may, however, be modified, or rather supplemented, so that it records two components of horizontal motion separately (on a moving surface) by attaching to one or other of the bobs a pair of slot guides at right angles to the direction of the two components, and pivotting in these the short ends of a pair of recording levers, so that each lever will be moved when the bob moves across the direction of the corresponding slot, but will not be moved when the bob moves along that direction. This makes a compact form of two-component horizontal seismograph, with the advantage that by retaining the ordinary index we have, in addition to the components, a plan drawn of the whole shaking. For the vertical component it is convenient to have a distinct astatically hung mass. But, as a sort of tour de force in astatic suspension, one or other of the bobs of the duplex pendulum may be allowed to have a limited amount of vertical freedom, and may have its equilibrium made nearly neutral for vertical displacements as well as for horizontal displacements. Let the upper bob, for instance, be hung from a platform which is free to rise and fall by rotating about a horizontal axis, and which is held up by springs. By applying the pull of the springs in such a manner that the moment of the pull about that axis is always nearly equal to the moment of the weight, we may approach vertical astaticism as closely as may be wished, and, provided the movements up and down are not too great to interfere with the proper gearing of the bobs, the mass will then possess universal freedom of translation, with nearly neutral equilibrium for all directions of displacement. In practical seismometry, however, it is no doubt advisable to restrict the freedom of the suspended mass to (at most) two degrees. The Society adjourned over the Long Vacation to Thursday, November 15th. * See 'Transactions of the Seismological Society of Japan,' vol. 3 (1881), p. 140, or the author's memoir on 66 'Earthquake Measurement" cited above. A complete three-component instrument is described in Nature,' vol. 34, p. 343. Transactions. Presents, June 21, 1888. Baltimore :-Johns Hopkins University. Studies from the Biological Laboratory. Vol. IV. No. 3. 8vo. Baltimore 1888. Batavia :-Bataviaasch The University. Genootschap van Kunsten en Wetenschappen. Tijdschrift voor Indische Taal- Land- en Volkenkunde. Deel XXXII. Aflev. 3. 8vo. Batavia 1888. Berlin:-K. Preuss. Akademie der Wissenschaften. berichte. 1888. Nos. 1-20. 8vo. Berlin. Physikalische Gesellschaft. Verhandlungen. Berlin 1885-86. The Society. SitzungsThe Academy. 1884-85. 8vo. The Society. Brussels:-Académie Royale de Médecine. Bulletin. Sér. 4. Tome II. Nos. 1-2. 8vo. Bruxelles 1888; Mémoires des Concours et des Savants Étrangers. Tome VIII. Fasc. 3. 4to. Bruxelles 1888. The Academy. Académie Royale des Sciences. Bulletin. Sér. 3. Tome XV. The Academy. Procès-verbal. Société Royale Malacologique de Belgique. Juillet-Décembre, 1887. 8vo. Bruxelles. Calcutta-Asiatic Society of Bengal. Descriptions of New Indian Lepidopterous Insects. By Frederic Moore. Part 3. 4to. Calcutta 1888. The Society. Cambridge, Mass.:-Harvard University. Bulletin. May, 1888. 8vo. [Cambridge.] The University. Catania:-Accademia Gioenia di Scienze Naturali. ProcessiVerbali. 1888. No. 3. 4to. [Catania.] The Academy. Essex Field Club:-The Essex Naturalist: Journal of the Essex Field Club. Vol. II. No. 4. 8vo. Buckhurst Hill 1888. The Club. Florence :-R. Biblioteca Nazionale Centrale. Bollettino delle Pubblicazioni Italiane. 1888. Gennaio-Giugno. 8vo. Firenze; Codici Palatini. Vol. I. Fasc. 7. 8vo. Roma 1888. The Library. Frankfurt-am-Main-Senckenbergische Naturforschende Gesellschaft. Abhandlungen. Bd. XV. Heft 2. 4to. Frankfurtam-Main. The Society. Frankfurt-am-Oder:-Naturwissenchaftlicher Verein des Regierungsbezirks Frankfurt. Monatliche Mittheilungen. Jahrg. V. Nr. 11-12. 8vo. Frankfurt-am-Oder 1888; Societatum. Litterae. 1887. No. 11. 1888. Nos. 3-4. 8vo. Frankfurtam-Oder. Hobart :-Royal Society of Tasmania. Nov. 21, 1887. 8vo. Hobart. The Verein. Abstract of Proceedings. The Society. |