trunk circuits, and a second series of ducts laid nearer the surface,
running into service boxes placed near together for lines to "house to
house" connections. In some cities where it is allowed to run overhead
lines, the plan of running but one service connection in a block is
followed, all customers in the block being supplied from a line run over
the housetops or strung on the rear walls.

This makes unnecessary all subsidiary ducts except a short one from the
manhole to the nearest building in the block, and effects a considerable
saving in pipe, service boxes, cables and labor. The manholes should
have their walls built up of brick, the floors should be of concrete,
and there should be an inside lid which can be fastened down and the
manhole thus made water-tight.

For ducts wood, iron or cement lined pipe may be used. To preserve the
wood it is generally treated with creosote, which, in contact with the
lead cover of the cable, sets up a chemical action, resulting in the
destruction of the lead. Wood offers but little protection for the
cable, as it is too easily damaged and broken through in the frequent
street openings made by companies operating lines of pipe in the
streets, and as one of the main purposes of a subway is that of a
protection to cables, wooden ducts have little to recommend them except
their cheapness.

Iron pipes are either laid in trenches filled in with earth or are laid
in cement. Iron pipe will of course rust out in time, and if absolute
permanence in construction is desired, should be laid in cement, for
after the pipe rusts out, the duct of cement is still left. However, if
we are going to the expense of laying in cement, it would be much
preferable to use cement lined pipe, which is not only cheaper than iron