=Law of Velocity.=—Next let us see what the velocity of the air is in the tube. Suppose that we have some convenient means of measuring the velocity of the air at any point, in feet per second or miles per hour, with some form of anemometer. We will have our measurements taken at the five points where we measured the pressure,—viz., at the tank, one-quarter, one-half, three-quarters and one mile from the tank. We will represent the velocities by a diagram similar to the one used for pressures. At the tank we find the air entering the tube with a velocity of 59.5 feet per second (40.6 miles per hour). We draw the vertical line M N, to represent this. At the quarter mile point the velocity is sixty-five feet per second (44.4 miles per hour) an increase in the first quarter of a mile of 5.5 feet per second. We construct the vertical line O P. At the half-mile point the velocity is 72.4 feet per second (49.4 miles per hour); at the three-quarter mile point it is eighty-three feet per second (56.8 miles per hour); and at the end of the tube, one mile from the tank, the air comes out of the tube with a velocity of 100.4 feet per second (68.5 miles per hour), about 1.7 times faster than it entered the tube at the tank. Drawing all the vertical lines to represent these velocities, and drawing a smooth curve line through the tops of our vertical lines, we have the curve of velocities, N, P, R, T, V, for all points along the tube. It is an increasing velocity and increases more rapidly as we approach the end of the tube. This is shown more clearly by drawing the straight dashed line N V. If the fluid flowing in the tube were inelastic, like water, then the curve of velocities would be a straight horizontal line, for the water would not come out of the tube any faster than it went in. But we are dealing with air, which is an elastic fluid, and, as we stated before, it expands as the pressure is reduced and becomes larger in volume. It is this expansion that increases its velocity as it flows along the tube. It must go faster and faster to make room to expand. Since the same actual quantity of air in pounds must come out of the tube each minute as enters the tube at the other end in the same time, to prevent an accumulation of air in the tube, and since it increases in volume as it flows through the tube, it follows that its velocity must increase.