with three points; the first of these is placed in a circuit, which is closed by the successive vibrations of the diaphragm; the second corresponds to an electric regulator, marking seconds on the cylinder; and the third subdivides the seconds there marked. Fig. 50 indicates a record thus made. In this case the obstacle is situated at a distance of sixty-two metres, and the vibration marks thirty-three oscillations per second. The interval occupied by two successive marks from the diaphragm on the paper corresponds to twelve oscillations, and the distance of the obstruction is then calculated by the following formula: D = 0.5 × 330 × 12/33 = 60 metres; so that the distance of the obstacle is recorded within two metres. [Illustration: FIG. 49. OBSTRUCTION-RECORDING APPARATUS.] [Illustration: FIG. 50. OBSTRUCTION-RECORDING APPARATUS.] “Amongst the special causes of accident may be mentioned the accidental absence of a piston to the train, breaking of the piston, the freezing up of a piston in the tube, and even forgetting the presence of a train, which has caused the entire service to be one train late throughout the day. Finally, the tubes themselves are sometimes broken or disturbed during street repairs, resulting of course in a complete cessation of traffic in the system till the damage is made good.”