pound of vapor, and after it is cooled down to sixty degrees it cannot hold it all, consequently the difference, or 0.00025 pound, must be deposited in the tube. Under these conditions one hundred thousand cubic feet of air will deposit twenty-five pounds of water in the tube. In the system of pneumatic tubes built and operated by the Batcheller Pneumatic Tube Company, the presence of a large quantity of moisture in the tube is prevented by using the same air over and over again. A little moisture may be deposited when the tube starts into operation, but the amount does not increase appreciably, as very little fresh air is admitted after starting. =Location of Obstructions in Tubes.=—In regard to the removal of obstructions in the tubes, I have had little or no experience; therefore under this heading I am satisfied to quote from the “Minutes of the Proceedings of the Institution of Civil Engineers,” London, Volume XLIII. “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