If the column of gas were stationary, the excess pressure of the gas over atmospheric pressure ought to be greater on the upper floors than on the lower (see problem 149). But when the gas is moving along the pipes, there is a fall in the pressure, owing to the action of the force of friction. Therefore the gas-pressure decreases as the gas gets farther from the source of supply. This decrease can be so great that the gas-pressure in the upper storeys is less than in the lower ones (since the gas is brought into the house by underground pipes).