At the stations considerable floor space or “yard room” would be required for side tracks, switches, etc. Usually the basement of a building would have to be utilized for the termination of such a tube. There are but few places in our large cities where the streets are so free from pipes, sewers, conduits, etc., that it would be practicable to build a thirty-six-inch pneumatic tube. When the service can be rendered by an eight-inch tube, the cost of installation favors its adoption. Steep grades cannot be ascended by these very large tubes, while the eight-inch tubes can be placed vertically. We do not say that there is no use for eighteen- and twenty-four-inch tubes, but the demand for them would be in special cases and we will not discuss them here. For ordinary mail and parcel service we recommend the use of six- and eight-inch tubes. An eight-inch carrier is twenty-four inches long, about seven inches inside diameter, and will contain five hundred ordinary letters. It weighs about thirteen pounds empty, and one can be despatched every six to ten seconds. We estimate that eighty per cent. of all the parcels delivered from a large retail department store could be wrapped up to go into these carriers. The minimum radius of curvature of an eight-inch tube is eight feet. =General Arrangement of Apparatus in the Stations. Two-Station, Two-Compressor Line.=—We will now proceed to a description of our system in detail. Figs. 24, 25, and 26 are diagrams showing how the tubes, air-compressor, tanks, sending and receiving apparatus are connected together at the stations. These diagrams are drawn to represent an eight-inch tube, but essentially the same arrangement would be used for smaller tubes. Fig. 24 represents a line of two stations with an air-compressor at each station. Such an arrangement is proposed for the line of postal tubes over the New York and Brooklyn bridge, or for any two stations located a very long distance apart, say six or eight miles. [Illustration: FIG. 24. DIAGRAM OF A TWO-STATION, TWO-COMPRESSOR LINE.]