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80 be made with sufficient length of horizontal connection to allow for this expansion. In Figures 31 and 32 the expansion of the mains and risers is taken up in the spring of the arms, AB. An old rule for the length of such expansion arms is that the length in feet should be equal to twice the diameter in inches. This, is a fair rule in most cases, but much depends on the amount of expansion to be taken care of, and no set rule can be given.

The most serious difficulties on account of expansion are met with in the long vertical risers of the modern high office buildings. In such cases any considerable movement of the riser is apt to result in trouble, as the radiator connections are generally short.

. 31 . 32 Provisions for Drainage and Expansion of Piping.

Various means are employed to overcome this. In buildings over ten stories in height it can generally be taken care of by anchoring the risers rigidly in the middle so they expand in both directions, and allowing for the expansion, by the connections to the supply main in the attic and to the returns in the basement. The radiators on the upper and lower floors, where most of the expansion takes place, must have connections sufficiently long to allow for it, and they must have sufficient pitch, so that they will not be trapped by the expansion of the risers. The author is familiar with one building 14 stories high in which expansion is entirely taken care of in this way. Radiators on the extreme floors