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 closing of the current by electrical contacts attached to diaphragms on which the sound wave acted. The contacts consisted of minute hammers resting on metal points fixed to the centre of diaphragms which closed the end of the experimental pipes. The signal marked the instant at which a sound wave impinged on a diaphragm. The markings on the paper band gave the period of time between two events, and the number of vibrations of the tuning-fork per second was estimated by means of markings due to the clock. The sound wave was usually originated by firing a pistol into the pipe furnished with diaphragms and contact pieces.

In the chronographic use of the Morse telegraph instrument (Stewart and Gee, Elementary Practical Phys. p. 234) a circuit is arranged which includes a seconds’ pendulum furnished with a fine platinum wire below the bob, which sweeps through a small mass of mercury forming a part of the circuit. There is a Morse key for closing the circuit. A fast-running Morse instrument and a battery are placed across this circuit as a shunt. A succession of dots is made on the paper ribbon by the circuit being closed by the pendulum, and the space between each adjacent dot indicates a period of one second’s duration. Also, when the key is depressed, a mark is made on the paper. To measure a period of time, the key is depressed at the beginning and end of the period, causing two dots to be made on the ribbon; the interval between these, when measured by the intervals due to the pendulum, gives the length of the period in seconds, and also in fractions of a second, when the seconds’ interval is subdivided into convenient equal parts. This apparatus has been used in determination of the velocity of sound. In the break circuit arrangement of pendulum key and Morse instrument the markings appear as breaks in a line which would otherwise be continuous. This combination was employed by Professors W. E. Ayrton and J. Perry in their determination of the acceleration of gravity at Tokio, 1877–1878 (Proc. Phys. Soc. Lond. 3, p. 268).

In the tuning-fork electro-chronograph attributed to Hipp a metal cylinder covered with smoked glazed paper is rotated uniformly by clockwork, a tuning-fork armed with a metallic style being so adjusted that it makes a clear fine line on the smoked paper. The tuning-fork is placed in the secondary circuit of an induction coil, so that when the primary circuit is broken an induced spark removes a speck of black from the paper and leaves a mark. The time period is deduced by counting the number of vibrations and fractions of vibration of the tuning-fork as recorded by a sinuous line on the cylinder. In later forms of this instrument the cylinder advances as it rotates, and a spiral line is traced. To obtain good results the spark must be very small, for when large it often leaps laterally from the end of the style, and does not give the true position of the style when the circuit is broken. The same arrangement of tuning-fork and revolving cylinder, with the addition of a standard clock, has been used by A. M. Mayer (Trans. Nat. Acad. Sci. U.S.A. vol. iii.) and others for calibrating tuning-forks, and comparing their vibrations directly with the beats of the pendulum of a standard clock the rate of which is known. The pendulum marks and breaks the primary circuit by carrying a small platinum wire through a small mercury meniscus. Better and apparently certain contacts can be obtained from platinum contact-pieces, brought together above the pendulum by means of a toothed wheel on the scape-wheel arbor. Sparking at the contact points is greatly reduced by placing a couple of lead plates in dilute sulphuric acid as a shunt across the battery circuit.

For Physiological Purposes.—A. Fick’s pendulum myograph or muscle-trace recorder is described in ''Vierteljahrsschr. der naturforsch. Ges. in Zürich, 1862, S. 307, and in Text-book of Physiology'', M. Foster, pp. 42, 45. It was used to obtain a record of the contraction of a muscle when stimulated. In many respects the instrument is similar to the electro-ballistic chronograph of Navez. A long pendulum, consisting of a braced metal frame, carries at its lower end a sheet of smoked glass. The pendulum swings about an axis supported by a wall bracket. Previous to an experiment, the pendulum is held on one side of its lowest position by a spring catch; when this is depressed it is free to swing. At the end of its swing it engages with another spring catch. In front of the moving glass plate a tuning-fork is fixed, also a lever actuated by the muscle to be electrically stimulated. When the pendulum swings through its arc, it knocks over the contact key in the primary circuit of an induction coil, the secondary of which is in connexion with the muscle. The smoked plate receives the traces of the style of the tuning-fork and of the lever attached to the muscle, and also the trace of an electromagnetic signal which marks the instant at which the primary circuit is broken. After the traces are made, they are ruled through with radial lines, cutting the three traces, and the time intervals between different parts of the muscle curve are measured in terms of the period of vibration of the tuning-fork, as in other chronographs in which the tuning-fork is employed.

In the spring myograph of E. Du Bois Reymond (Munk’s Physiologie des Menschen, p. 398) a smoked glass plate attached to a metal rod is shot by a spiral spring along two guides with a velocity which is not uniform. The traces of a style moved by the muscle under examination, and of a tuning-fork, are recorded on the glass plate, the shooter during its traverse knocking over one or more electric keys, which break the primary circuit of an induction coil, the induced current stimulating the muscle.

In the photo-electric chronograph devised by G. J. Burch, F.R.S. (Journ. of Physiology, 18, p. 125; Electrician, 37, p. 436), the rapid movements of the column of mercury in a capillary electrometer used in physiological research are recorded on a sensitive plate moving at a uniform angular velocity. The trace of the vibrating prongs of a tuning-fork of known period is also recorded on the plate, the light used being that of the electric arc. The images of the meniscus of the mercury column and of the moving fork are focused on the plate by a lens. Excellent results have been obtained with this instrument.

An important development of a branch of chronography is due to E. J. Marey (Comptes rendus, 7. août 1882, and Le Mouvement, par E. J. Marey, Paris, 1894), who employed a photographic plate for receiving successive pictures of moving objects, at definite times, when investigating the movements of animals, birds, fishes, insects, and also microscopic objects such as vorticellae. The instrument in one of its forms consisted of a camera and lens. In front of the sensitive plate and close to it a disk, pierced with radial slits, revolved at a given angular velocity, and each time a slit passed by the plate was exposed. But since, in the time of passage of the space between the slits, the object had moved by a certain amount across the field of view, a fresh impression was produced at each exposure. The object, well illuminated by sunlight, moved in front of a black background. Since the angular velocity of the disk was known, and the number of slits, the time between the successive positions of the object was also known.

Marey (La Méthode graphique, pp. 133, 142, 456), by means of pneumatic signals and a rotating cylinder covered with smoked glazed paper, measured the time of the movements of the limbs of animals. The instrument consists of a recording cylinder rotated at a uniform angular velocity by clockwork controlled by a fan governor, and pneumatic signal, constructed thus. One end of a closed shallow cylinder, about 4 cm. dia., is furnished with a stretched rubber membrane. A light lever, moving about an axis near the edge of the cylinder, is attached to the centre of the membrane by a short rod, its free end moving as the membrane is distended. The cylinder is connected by a flexible tube with a similar cylinder and membrane, but without a lever, which is attached to that part of the body of the animal the movement of which is under investigation. The system is full of air, so that when the membrane attached to the animal is compressed, the membrane which moves the lever is distended and the lever moved. Its end, which carries a scribing point, marks the smoked paper on the rotating cylinder. The pneumatic signal is called by Marey “tambour à levier.”

References to Chronographic Methods:—(1) Chronographs used in Physiology: Helmholtz, “On Methods of measuring very small Portions of Time,” ''Phil. Mag. (1853), 6; Id., Verhandlungen der physikalisch-medicinischen Gesellschaft in Würzburg (1872); Harless, “Das Attwood’sche Myographion,” Abhandlungen der k. bayerischen Akademie der Wissenschaften (1862); Id., Fall-Myographion aufgestellt in der Wiener Weltausstellung in der Abteilung für das Unterrichtswesen von Ungarn (Budapest, 1873); Hensen, “Myographion mit vibratorischer Bewegung,” Arbeiten aus dem Kieler physiol. Instit. (1868); Brücke, Sitzungsber. d Wien. Acad. (1877); Pflüger, “Myographion ohne Bewegung,” Untersuchungen über die Physiologie des Electrotonus (1859); Pouillet, Compt. rend. (1844); I. Munk, Physiologie des Menschen (for Pflüger’s cylinder governed by conical pendulum); J. G. M’Kendrick, Life in Motion (1892) (for early form of cylinder chronograph by Thomas Young); Stirling, Outlines of Practical Physiology'' (for reaction-time chronographs of F. Galton and Exner). (2) Chronographs used in gun work and for other purposes: Sabine, ''Phil. Mag. (1876); Moisson, Notice sur la chronographie système Schultz (Paris, 1875); Paul la Cour, La Roue phonique (Copenhagen, 1878); Mach, “Collected Papers on Chronographs,” Nature, 42, p. 250; C. V. Boys, “Bullets photographed in Flight,” Nature, 47, p. 415; Pneumatic Tube Co., Paris, “Chronograph,” Nature, 9, p. 105; G. C. Foster, “Laboratory Chronograph,” Nature, 13, p. 139; E. S. Holden, “Astronomical Chronograph,” Nature, 26, p. 368; D’Arsonval, La Lumière électrique (1887); Dunn, “The Photo-retardograph,” Journal United States Artillery, 8, p. 29; E. J. Marey, La Méthode graphique (for Deprez accélérographe); Werner Siemens, “Electric Spark Chronograph,” Wied. Ann.'' (1845), 66.

 CHRONOLOGY (Gr. , computation of time,  ), the science which treats of time, its object being to arrange and exhibit the various events which have occurred in the history of the world in the order of their succession, and to ascertain the intervals of time between them. The term “chronology” is also used of the order in time itself, as adopted, and of the system by which the order is fixed.

The preservation of any record, however rude, of the lapse of time implies some knowledge of the celestial motions, by which alone time can be accurately measured, and some advancement in the arts of civilized life, which could be attained only by the accumulated experience of many generations (see ). Before