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 tank. The attic mains are drained directly into the supply risers, which drop from the bottom of them.

In connecting up radiators on a one-pipe system they should be set so as to pitch slightly toward the connection from the riser, and the connection should always pitch toward the riser. Con- nections which, on account of carelessness in workmanship, were pitched in the opposite way, are a very fertile cause of water- hammer. Eadiators set on two-pipe systems should be pitched slightly toward the return connection, which should not be con- nected from the same end as the supply. There are a number of plants in which radiators connected on two-pipe systems have both the supply and return connections at the same end of the radiator, but in the opinion of the author this is a very bad prac- tice, as the radiator might very much better be connected on the one-pipe system. In fact, when radiators with such connections are in operation, unless the return connection is lower than the supply, which is not generally the case, the water of condensation is just as apt to Tun down the supply connection as down the re- turn. Furthermore, when such radiators are turned on, if the sup- ply valve is opened first, any water which may be in the radiator runs out of this connection, as well as the large amount that is formed by the first contact of steam with the cold radiator; and if the return valve is opened first, the water in the return pipe backs up into the radiator, so that when the supply valve is opened, a large amount of it will run out to the supply connec- tion. At this point it should be stated that in turning on radia- tors with two-pipe connections the supply valve should always be opened first; and the fact that the uninformed occupants of rooms frequently do not know which is the supply valve is one of the objections of two-pipe systems.

Expansion of pipes. The expansion of pipes is an important consideration in any case, and where there are long mains or in high office buildings, which consequently have long vertical risers, it becomes a consideration of vital importance. The coefficient of expansion of wrought-iron pipe is 0.000007 per degree Fahr. This amounts to about 1.5 inches in a 100-foot length for low-pressure steam pipes. In horizontal mains this can be gen- erally taken care of by making turns or offsets in the mains in every 50 or 75 feet of pipe, the expansion being taken up by the spring of the pipe. All connections from mains or risers should