The practical question in gas distribution is, what quantity of gas will a given actuating pressure cause to flow along a pipe of given length and given diameter? The solution of this question allows of the diameters of pipes being arranged so that they will carry a required quantity of gas a given distance under the actuating pressure that is most convenient or appropriate. There are five quantities to be dealt with, viz.: (1) The length of pipe = _l_ feet. (2) The internal diameter of the pipe = _d_ inches. (3) The actuating pressure = _h_ inches of head of water. (4) The specific gravity or density of the gas = _d_ times that of air. (5) The quantity of gas passing through the pipe--Q cubic feet per hour. This quantity is the product of the mean velocity of the gas in the pipe and the area of the pipe. The only work done in maintaining the flow of gas along a pipe is that required to overcome the friction of the gas on the walls of the pipe, or, rather, the consequential friction of the gas on itself, and the laws which regulate such friction have not been very exhaustively investigated. Pole pointed out, however, that the existing knowledge on the point at the time he wrote would serve for the purpose of determining the proper sizes of gas-mains. He stated that the friction (1) is proportional to the area of rubbing surface (viz., pi_ld_); (2) varies with the velocity, in some ratio greater than the first power, but usually taken as the square; and (3) is assumed to be proportional to the specific gravity of the fluid (viz., _s_). Thus the force (_f_) necessary to maintain the motion of the gas in the pipe is seen to vary (1) as pi_ld_, of which pi is a constant; (2) as _v^2_, where _v_ = the velocity in feet per hour; and (3) as _s_. Hence, combining these and deleting the constant pi, it appears that _f_ varies as _ldsv^2_.