If a larger than eight-inch tube is to be used for mail service, it should be not less than thirty-six inches. Carriers larger than eight inches cannot be handled: they are too heavy. They are also too heavy to slide through the tube, hence, must be mounted upon wheels. It is not practical to make a carrier on wheels less than eighteen or twenty-four inches, and the carrier must be at least twenty-four inches to contain a large mail-pouch. Now, if we are going to despatch mail-pouches through a pneumatic tube we must send more than one in a carrier, otherwise the service will be too slow. Such large carriers could not be despatched oftener than once or at most twice in a minute. Suppose we were to transport the mail from a railway station to a main post-office. A train arrives with, say, sixty pouches. If only one pouch could be put into a carrier and the carriers could be despatched at half-minute intervals, it would take thirty minutes to despatch all the pouches. Now, suppose we make the tube thirty-six inches. The carriers will be eight feet long and will contain from twelve to fifteen pouches. Five carriers would contain the entire train-load of mail, and they could be despatched in four or five minutes. [Illustration: FIG. 22. CROSS-SECTION OF A 36-INCH TUBE.] [Illustration: FIG. 23. CARRIER FOR A 36-INCH TUBE.] =System of Very Large Tubes.=—The cross-section of a thirty-six-inch tube is shown in Fig. 22. It is built flat on the bottom and sides, with an arched top. The floor is of concrete containing creosoted ties; the side walls and top are of brick, plastered with cement upon the interior. The two tubes may be built one above the other or side by side, depending upon the condition of the streets, but one common separating wall will serve for both. The carriers, one of which is shown in Fig. 23, run on two rails laid close to the sides of the tube. At curves a guard-rail is placed upon the side wall, making it impossible for a carrier to leave the track. The carriers are made of hard wood with an iron frame, and are as light as consistent with the service required of them. They are open on top. Their outside dimensions are thirty-four inches by thirty-four inches by eight feet. The sending and receiving apparatus for these very large tubes have to be specially designed for each particular station, so no attempt will be made here to describe them. The air-pressure required depends upon the length of the line. If it were not more than six or eight ounces a fan would be used to maintain the air-current, but for pressures above this, up to a pound or two per square inch, some form of positive blower would be used.