=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.