Now we will draw a smooth curve through the tops of all our vertical lines, and we have a curve, E, G, I, K, L, representing the pressure in the tube at every point. It falls gradually from ten pounds to zero, but it does not fall in exact proportion to the distance from the tank. Such a fall of pressure would be represented by the straight dash-line, E, L. The reason why the true pressure-curve is not a straight line, and lies above a straight line, is because air is an elastic fluid and expands, becoming larger in volume as the pressure diminishes. The straight dash-line represents the fall of pressure of an inelastic fluid, like water, when flowing in the tube. The fall of pressure along the tube is analogous to the fall of level along a flowing stream. In fact, we frequently speak of the descent of a stream as the “head of water” when it is used for power purposes, and we mean by this the pressure the water would exert if it were confined in a pipe. The descent, or change of level, in the bed of a stream is necessary to keep the water flowing against the friction of the banks. The descent of the water imparts energy to overcome the friction. In a similar manner, we must have a fall of pressure along the pneumatic tube to overcome the friction of the air against the interior surface of the tube. We find another analogue in the flow of the electric current along a wire; here there is a fall of potential necessary to overcome the resistance of the wire. Since power has to be expended to compress the air and impart to it its pressure, when this pressure disappears we know that the air must be losing its energy or doing work, and we look to see what becomes of it. In the present case, we find that most of this work is expended in overcoming the friction between the air and the surface of the tube. =Uses of Pressure Curves.=—The pressure curve teaches us many things. Suppose we were to establish stations on this tube at the quarter, half, three-quarter, and mile points; we see at once that intermediate-station or closed receivers, described in the last chapter, must be used at all of the stations except the mile point at the end of the tube, because the pressure in the tube is so high above the pressure of the atmosphere that we could not open the tube to let the carriers come out, but at the end of the tube we could use the open receiver. In designing our sending and receiving apparatus for each station, we look to this pressure curve to tell us the pressure which we shall have on the pistons in our cylinders, and are thereby enabled to make them with proper proportions for the work that they have to do.