=Intermittent and Constant Air-Current.=—Having thus defined a pneumatic tube, there are two ways in which we may operate it to transport our carriers containing mail, packages, or other matter. The first method consists in storing our compressed air in a suitable tank, or by exhausting the air from the tank; then, when we wish to despatch a carrier we place it in the tube and connect the tube with the tank by opening a valve. As soon as the carrier arrives at the distant end of the tube the valve is closed and the air soon ceases to flow. When a long interval of time elapses between the despatching of carriers, this is the most economical method of operation, but usually carriers have to be despatched so frequently that a great deal of time would be lost if the air-current had to be started and stopped for each carrier. The second and more usual method of operation consists in maintaining a constant current of air in the tube and in having the carriers inserted and ejected at the ends of the tube without stopping the current of air for any appreciable length of time. It is analogous to launching boats in a rapidly flowing stream, allowing them to float down stream and then withdrawing them. When the boats are in the stream they present little obstruction to the flow of water and check its speed but very little. In order to compute the speed with which the boat will pass from one point to another, we only have to know the speed of the stream between those points when no boat is in it. The presence of the boat does not change the speed appreciably. So it is with carriers in a pneumatic tube: they are carried along with the current of air. The air flows nearly as rapidly when a carrier is in the tube as when there is none. The friction of the carrier against the inner surface of the tube creates a slight drag, but it checks the speed of the air only a little. Therefore, in order to know the speed with which a carrier will be transported from one station to another through a pneumatic tube, we need only to know the velocity with which the air flows through the tube when no carrier is present. Of course there are special cases of heavy carriers, or carriers having a large amount of friction from their packing, or of tubes not laid horizontally, where the resistance of the carrier must be taken into consideration, but for our present purpose we will neglect all of these conditions. =Laws Governing the Flow of Air in Long Tubes.=—This leads us to study the laws governing the flow of air in long tubes, omitting for the present the presence of a carrier. Since tubes operated intermittently have become obsolete, we will only consider the case of a constant current of air, this being what we have to deal with in practice. [Illustration: FIG. 48. PRESSURE AND VELOCITY CURVES.]