From a mechanical point of view the system has been brought to such perfection that it is no more experimental than a locomotive or an electric tramcar. The unique value of tube service is due to immediate despatch, high velocity of transit, immunity from traffic interruption, and economy. The greatest obstacle to rapid intercommunication is the delay resulting from accumulations due to time schedules. The function of tube service is to abolish time schedules and all consequent delays.
The number of trades parcels annually delivered in London is estimated at more than 200,000,000. A careful canvass has been made of 1,000 shops only, which represent a very small fraction of the total number in the county. As a result it has been ascertained that these 1,000 shops deliver no fewer than 60,000,000 parcels yearly, a fact that seems to more than justify the foregoing estimate; on the other hand, it is known from official data that the parcel post in London is represented by less than 25,000,000, or one-ninth of the total parcel traffic. With a tube system in operation, every parcel, instead of waiting for "the next delivery," would leave the shop immediately. After being despatched by the tube it would be delivered at a tube station within a quarter of a mile at least of its destination, and thence by messenger. The entire time consumed for an ordinary parcel would be not over an hour, and for a special parcel fifteen to twenty minutes. They require from three to six hours or longer at present.
The advantages of the tube system to the public would be manifold. Customers would find their purchases at home upon their return, or, if they preferred, could do their shopping by telephone, making their selections from goods sent on approval by tube. The shopman would find himself relieved from a vast amount of confusion and annoyance, less of his shop space given up to delivery, and his expenses reduced. Small shops would be able to draw upon wholesale houses for goods not in stock, while the customer waited. Such delay and confusion as are frequently occasioned by fogs would be reduced to a minimum.
While the success of the project is not dependent on Post Office support, the Post Office should be one of the greatest gainers by it. The time of delivery of local letters would be reduced from an average of three hours and six minutes to one hour. Express letters would be delivered more quickly than telegrams. This has been demonstrated conclusively again and again in New York and other American cities where the tubes have been in operation for years. The latest time of posting country letters would be deferred from one-half to one hour, and incoming letters would be advanced by a similar period. The parcels post would gain in precisely the same way, but to an even larger extent.
If the Post Office choose to avail themselves of the opportunity, every post office will become a tube station and every tube station a post office. Thus the same number of postmen covering but a tithe of the present distances could make deliveries without time schedules at intervals of a few minutes with a handful instead of a bagful of letters.
The sorting of mails would be performed at every station instead of at a few. Incoming country mails would be taken from the bags at the railway termini, and the same bags refilled with outgoing country mails, thus avoiding needless carriage to the Post Office and back. No bags at all would be used for local mails, the steel carriers themselves answering that purpose.
At every tube terminal a post-office clerk would be stationed, so that the mails would never for an instant be out of post-office control. Its absolute security would be further ensured by a system of locking, so that the carriers could only be opened by authorised persons at the station to which they were directed. These safeguards offer a striking contrast to the present method that entrusts mail bags to the sole custody of van drivers in the employ of private contractors.
If the mails were handled by tube, business men would be able to communicate with each other and receive replies several times in one day, and country and foreign letters could always be answered upon the day of receipt. The effect would be felt all over the Empire.
Would the laying of the tubes seriously impede traffic? The promoters assure us that the inconvenience would not be comparable to that caused by laying a gas, water, or telephone system. When one of those has been laid the annoyance, they urge, has only begun. The streets must be periodically reopened for the purpose of making thousands of house connections, extensions, and repairs. When a pneumatic tube is once down it is good for a generation at least. It is not subject to recurrent alterations incidental to house connections and repairs. In three American cities the tubes have been touched but three times in twelve years, and in those cases the causes were a bursting water main and faulty adjacent electric installations. The repairs were effected in a few hours.
From a general consideration of the scheme we may now turn to some mechanical details. The pipes would be of 1 foot internal diameter, made in 12-foot lengths. "Straight sections," writes an engineering correspondent of The Times, "would be of cast-iron, bored, counter-bored, and turned to a slight taper at one end, to fit a recess at the other end (of the next tube), to form the joints, which could be caulked. Joints made in this way are estimated to permit of a deflection of 2 inches from the straight, so that the laying and bedding need not be exact. Bent sections are to be of seamless brass; these are bored true before bending. The permissible curvature is determined upon the basis of a maximum bend of 1 foot radius for every 1 inch of diameter; the 1 foot diameter of the London tubes would consequently be allowed a maximum curvature of 12 foot radius. Measured at the enlarged end, the over-all diameter of each pipe is 17 inches, and as two such pipes are to be laid side by side, with 18 inches between centres, the clear width will be 35 inches. The trenches are therefore to be cut 36 inches wide, and in order to have a comparatively free run for the sections, it is proposed to cut the trenches 6 feet deep."