The branch and switch system has many attractions for the inventor, and upon first thought it would seem the most feasible solution of the problem. It has been the dream of more than one inventor, as the records of the patent-office show, but no one has succeeded in working it out. The current of air cannot be divided; carriers passing from the branch into the main line must not collide with other carriers running in the main line; a certain minimum distance must always be maintained between the carriers in the same tube; when a carrier is despatched it must go directly to the station for which it is intended without further attention from the sender and it must not interfere with other carriers; expense of manufacture prohibits the use of any but round smooth tubes up to eight inches in diameter, hence projections cannot be placed upon the carrier to give it an individuality and cause it to operate a switch at any particular point along the line; the carrier is free to rotate in a round tube about its longitudinal axis, therefore, its individuality must be indicated by some symmetrical marking about this axis, if it is to be automatic in its operation; the speed of the carrier is so high that electrical contacts placed in distinctive positions on the carriers cannot be used while it is in motion, for mechanism having inertia could not be moved during the short time that the electric circuit would be closed; only the simplest attachments can be made to the carrier, for constructional reasons and because of the rough usage that they receive. These and numerous other reasons make the problem most difficult. We have not attempted to solve it by the use of branch tubes and electrically operated switches, but have adopted the simpler and equally effective method of carrying the main line through each of the stations that it unites. In our system each carrier has an individuality determining the station at which it will be discharged from the tube. By a simple attachment to, the front end of the carrier, consisting of a circular metal disk, the sender so marks the carrier that it will pass all stations until it arrives at the station for which it was destined and will there pass out of the tube. In addition to this a method has been devised whereby carriers can be inserted into the tube without the possibility of collision with carriers already running in the tube.
Fig. 21.
A DIAGRAM SHOWING VARIOUS METHODS OF CONNECTING THE STATIONS OF A LARGE SYSTEM WITH PNEUMATIC TUBES.
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Referring now to the diagram, Fig. 21, we have here an imaginary system which we will suppose to be located in some large city. The two large squares I and II indicate central pumping stations, and the small squares A, B, C, D, etc., indicate receiving and sending stations. Some of the stations, such as A, B, C, D, E, F, and Y, which do a large amount of business and may be supposed to be large retail stores, are connected directly with the central station by double tubes, one for sending and the other for receiving carriers. Two smaller stores, such as G and H, may be located on the same line. At I, J, K, and L we have four stations, all connected by the same double line of tubes. These stations we will imagine to be located in the residence section of the city. Carriers containing parcels of merchandise or other matter destined for private residences would be sent from the stores A, B, C, etc., to the central station I, where they would be transferred to the line 2 and be adjusted to stop at the station nearest the residence to which the parcels were addressed. From this station the parcels would be delivered by messengers to the residences. If a carrier is to be sent from the central station I to station K, it will be so adjusted before it is put into the tube that it will pass stations I and J, but be discharged automatically from the tube when it arrives at station K. In a similar manner carriers can be despatched from station L to station I or from station J to station L. In passing through the central station the carriers are manually transferred from one line to another.
In another part of the city we may have another central pumping station, II; and the two central stations may be connected by a double trunk line, 3. Again, we have lines radiating from this central station, as shown by station Y. There will be some localities where it will be an advantage to arrange the stations upon a loop, as shown in circuit 4, where stations S, T, U, V, W, and X are connected together in this way. Or we can combine the two arrangements of loop and direct line, as shown in circuit 5. Stations O and R are on the double line, but from O a loop is formed including stations N, M, P, and Q. Here it is supposed that the stations O and R do a much larger business with the central station II than the stations N, M, P, and Q, this being the principal reason for placing them on the double line. All carriers must be returned to the station from which they were sent, or others to replace them, otherwise there will be an accumulation of carriers at some of the stations. It is like a railway: there must be as many trains despatched in one direction as the other, each day. Station O can receive a carrier from the central station and return it directly, but when station N receives one it must be returned via M, P, Q, O, and R, a much longer route than that by which it was received. This disadvantage is compensated, when stations N, M, P, and Q do only a small amount of business, by the less cost of laying a single line. If a carrier is to be sent from M to N, it must go via P, Q, and O, being manually transferred at O from the “down” to the “up” line. P can send directly to Q, but Q must send to P via O, N, and M. R can send directly to O and O to R. Similarly in circuit 4 the carriers must all travel around the loop in the same direction, shown by the arrows. Station S can receive carriers directly from the central station, but they must return via U, W, X, V, and T.
Again, we may have a double-loop line, as indicated in the diagram by circuit 6. Here five stations, a, b, c, d, and e, are connected by a loop consisting of two lines of tube, in which the air circulates in one direction in one line and in the opposite direction in the other. Here b can send directly to c, c directly to b, and e to b via d and c, or via central and a. This is an arrangement that would be used where there is a large amount of business between the stations on the loop. As stated before, the best arrangement for any particular locality depends entirely upon circumstances.
Size of Tubes.
—The pneumatic-tube system that we are describing is not limited to any particular size of tube. The size is usually determined by the number and size of packages to be transported. A small tube, two or three inches in diameter, is best suited for telegrams and messages; mail, parcels, etc., require a six- or eight-inch tube, while mail pouches and bulky material, a thirty-six inch or possibly larger tube. We divide tubes into three classes, according to their size, naming them small, large, and very large tubes. By small tubes we mean those not larger than three or possibly four inches in diameter. Large tubes are those having a diameter more than four inches and not more than eight inches. Very large tubes include all that are more than eight inches. This classification is for convenience, but it has a deeper significance. For example, in the transportation of mail, it must either be handled in bulk, that is, in pouches, or in broken-bulk, that is, loose or tied up in small packages. There are many advantages in transporting it in broken-bulk, in fact, there are very few places where it could be handled in any other way. For this service six- or eight-inch tubes—not larger—are best suited. The carriers are light enough to be easily handled; they are not so large in capacity as to make it necessary to wait for an accumulation of mail to fill them; they can be delivered from the tube on to tables at any point in the building where the mail is wanted, for cancelling, distribution, or pouching, thus rendering a very rapid service; the mail is kept moving in an almost constant stream, keeping the postal employees more uniformly employed; special carriers can be despatched with “special delivery” letters. In other words, the most rapid service can be rendered by this size of tube.