The expansion of water in these apparatus never exceeds 1 in 30; that is, the top water line in the apparatus never stands higher than the cold-water line in the cistern which feeds it, more than lft. for each 33ft. vertical height to which the apparatus extends. It is seldom that an apparatus of this kind exceeds 60 ft. vei'tical height, and at this height it is seldom that the water is anywhere near boiling point in every part of the apparatus (except the cold supply pipe), as the 1 in 30 rule requires it to be. The common practice, therefore, is to let the expansion pipe extend at least 2 ft. above the cold-water line in houses of moderate size, and 3 ft. or more in tall houses. This is easily remembered, quite safe, and applies to all systems of apparatus. The quantity of water held in the apparatus makes no theoretical difference In practice, it may mean that the large quantity does not get so hot.