In computing the capacity of pipes used to convey liquids one should remember that the capacity varies with the area, and that the areas of similar figures vary as the squares of their corresponding dimensions. Pipes being cylindrical in shape are, therefore, similar figures. The areas of any two pipes are to each other as the squares of their diameters.

Thus, if one pipe is 6 in. in diameter, and another is 3 in. in di-anieter, their ratio is 36 / 9, or 4 to 1, and the area of the larger one is, therefore, 4 times that of the smaller one.