that when a carrier is despatched a weight is raised and allowed to fall, carrying with it a piston in a cylinder filled with oil. While the weight is rising and falling the sending apparatus is locked, but becomes unlocked when the weight is all the way down. A by-pass in the cylinder permits the oil to flow from one side of the piston to the other, and the size of this by-pass can be regulated, thus determining the time that the weight shall take in descending. This makes a simple and effective time-lock that does not get out of order. The time required for a carrier to travel from the main to the sub-post-office is sixty seconds, and from the sub- to the main post-office, fifty-five seconds. This difference of time in going and returning is due to the expansion of the air in the tube, as will be explained more fully in another place. The distance between the offices being two thousand nine hundred and seventy-four feet, gives an average speed of about fifty-two feet per second, or 35.27 miles per hour. Of course the speed can be increased by increasing the air-pressure, but this speed is found in practice to be ample for all requirements. In order to give some idea of the energy possessed by one of these carriers travelling at this speed, it may be said that if the end of the tube were left open and turned upward, an emerging carrier would rise about forty feet into the air. It is easy to imagine how apparatus, depending for its operation upon impact with a moving carrier, would be soon destroyed, as well as the carriers themselves. This is why receiving apparatus used with small tubes and light carriers cannot be applied to large tubes with heavy carriers. No serious trouble has ever been experienced from carriers getting wedged in any part of these tubes.