“Intimately connected with the working of the tubes is the removal of obstructions which occur from time to time, causing not unfrequently serious inconvenience and delay. The most general cause of obstruction is a stoppage of the train arising from accident to the tube, to the carriers or piston, or to the transmitting apparatus. In such cases the delay is generally very brief, it being for the most part sufficient to reverse the pressure on the train from the next station, and to drive it back to the point it started from. If one or more of the carriers break in the tube, reverse pressure is also generally sufficient to remove the obstacle; but where this fails, the point of obstruction must be ascertained. This is done by carefully observing the variations of air pressure in the reservoir when placed in connection, first with a line of known length, and then with the obstructed tube. By this means the position of the obstruction can be ascertained within one hundred feet. Or the tube may be probed with a long rod up to a length of two hundred feet. A very ingenious apparatus, by M. Ch. Bontemps, is shown in Figs. 49 and 50, and is employed to ascertain the exact position of the obstruction. It acts by the reflection of sound-waves on a rubber diaphragm. A small metal disk is cemented to the rubber, and above this is a pointed screw, D. An electric circuit is closed where the points C and D are brought in contact. To locate an obstruction a pistol is fired into the tube as shown, and the resulting wave, traversing the tube at the rate of three hundred and thirty metres a second, strikes the obstruction and is then reflected against the diaphragm, which in its turn reflects it to the obstacle, whence it returns to the diaphragm. By this means indications are marked on the recording cylinder, and if the interval of time between the first and second indications be recorded, the distance of the obstacle from the membrane is easily ascertained. The chronograph employed is provided 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.
Fig. 49.
OBSTRUCTION-RECORDING APPARATUS.
Larger image (359 kB)
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.”