In order to make our ideas and thoughts as clear as possible let us represent them by a diagram, Fig. 48. We will suppose that a tank, A, is kept constantly filled with compressed air at a pressure of ten pounds per square inch, from some source of supply. We will suppose that the pressure of the air in this tank never changes, air being supplied as fast as it flows away. Next, let us assume that a tube eight inches in diameter inside and one mile long (five thousand two hundred and eighty feet) is connected to the tank at one end and left open to the atmosphere at the other. The air will flow in a constant stream from the tank into the atmosphere, for the reason that air is being supplied to the tank as fast as it flows away. =Law of Pressure.=—First, let us consider the pressure of the air at various points in the tube. We will, for convenience, represent the pressure in the tank by a vertical line, D E, ten units in length, since the pressure is ten pounds per square inch. Now let us go to a point on the tube one quarter of a mile (one thousand three hundred and twenty feet) from the tank, drill a hole in the tube, attach a pressure-gauge and measure the pressure of the air at this point. We shall find it to be about 7.91 pounds per square inch; or, 2.09 pounds below the pressure in the tank. We will represent this on our diagram by another vertical line, F G, having a length of 7.91 units. Again let us measure the pressure in the tube at a point one-half a mile (two thousand six hundred and forty feet) from the tank. Here we find it to be about 5.61 pounds per square inch, and we represent it by the vertical line, H I, having 5.61 units of length. We note that the pressure is 4.39 pounds below the pressure in the tank. We are at the middle point of the tube and the pressure has fallen to nearly, but not quite, one-half the pressure in the tank. We will now go to a point three-quarters of a mile (three thousand nine hundred and sixty feet) from the tank, and here the pressure is about 3.01 pounds per square inch. We represent it by the vertical line, J K. Lastly, we measure the pressure very near the end of the tube, one mile from the tank, and find it to be about zero, or the same as the pressure of the atmosphere. All of our measurements have been in pounds above the atmospheric pressure; to express them in absolute pressure, we should add to each the pressure of the atmosphere, which is 14.69 pounds, nearly.