Operation of the Tubes.
—The tubes are kept in constant operation during the day, and six days of the week. The air-compressor is started at nine o’clock in the morning and runs until seven in the evening, except during the noon hour, the air flowing in a constant steady current through the tubes. When a carrier is placed in the tube it is carried along in the current without appreciably affecting the load on the compressor. Carriers may be despatched at six-second intervals, and when they are despatched thus frequently at each office, there will be eighteen carriers in the tube at the same time. If ten carriers per minute are despatched from each office, and each carrier contains two hundred letters, the tube has a carrying capacity of two hundred and forty thousand letters per hour, which is far beyond the requirements of this office. About five hundred carriers a day are despatched from each office. This varies considerably on different days and at different seasons of the year. Experience has taught that a certain period of time should elapse between the despatching of carriers, in order that they may not come in contact with each other, and that the receivers may have time to act. With the present plant this period is made about six seconds. In order to make it impossible for carriers to be despatched more frequently than this, time-locks are attached to the sending apparatus. One of these locks may be seen in Fig. 13, connected to the handle of the sending apparatus. It is so arranged 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.
Benefits of the System.
—The advantages to the post-office department by the adoption of this system have been numerous, and the post-office officials who are familiar with the operation of the tubes frequently speak in high terms of their usefulness. Formerly the mail was transported from one office to the other by a wagon making a trip every half-hour. Considerable time has been saved by the greater speed of transit, but even more time is gained by keeping the mail moving instead of allowing it to accumulate and then despatching it in bulk. With the pneumatic system a letter posted in the sub-post-office will reach its destination just as quickly as if posted at the main office, and sometimes more quickly. Let us take an example, first, with the old wagon service. Suppose that you drop a letter in the sub-post-office; it lies there, say, fifteen minutes waiting for the departure of the next wagon; it is put into a pouch with hundreds of other letters, and ten minutes are consumed in transporting it to the main office. When it arrives there the pouch is thrown on the floor at the entrance of the building; in a few minutes, more or less, a clerk takes the pouch, throws it on a truck and wheels it around to the cancelling machines, where it may lie for five or ten minutes more before being opened, and then perhaps five minutes will elapse before your letter reaches the cancelling machine. It would not be unusual for three-quarters of an hour to elapse from the time you dropped your letter in the office until it was cancelled. Now with the pneumatic tube service forty minutes of this time will be saved; for immediately after you drop your letter in the office it will be despatched through the tube and delivered on the table in front of the cancelling machines. Soon after the tubes were installed the postmaster’s attention was called to an instance where letters from the sub-office were sent through the tube and were despatched to New York City one train earlier than they could have been had the old wagon service been in use. People frequently post letters requesting that they be sent through the tube; of course they would be sent in that way if the request was not made, but it shows that the public recognize the better service. Formerly mail was collected from the street boxes in the banking section of the city and the collectors carried it to the main office. After the tubes were installed this mail was carried to the sub-post-office to be sent through the tube, and the time formerly occupied in walking to the main office was then utilized in having the men face up the letters ready for the cancelling machines,—a double saving in time besides making their labor much lighter and enabling them to do more useful work.
Since the sub-office has been established in the Bourse, it has been made a distributing as well as receiving office. At least two more deliveries of mail are made each day in the Bourse building than in any other office building in the city.
All letters mailed in the sub-office with a special delivery stamp are despatched through the tube immediately.
It is now nearly four years since the system was put into operation. During that time more than thirty-five million letters have been transported, and all the repairs to the system have not required it to be stopped for more than a few hours. During the first year the Pneumatic Transit Company operated the tubes at their own expense, agreeing at the end of that time to take them out if the government so requested. Since the first year the government has paid the running expenses.
Such is the history of the first United States pneumatic postal system. Such is the history of the first pneumatic tubes of sufficient size to carry all the first class and most of the lower classes of mail, in this or any other country, so far as the writer knows.