the duct step, against which the ducts were laid, recesses were provided
for the projecting bond. This was done by nailing to the rough shutters
for the steps a form which when removed left a dove-tailed vertical
groove. This form was made in two pieces, one tapering inward and the
other with more taper outward. As the bonds were placed, these grooves
were filled with mortar.

The ducts usually received their final rodding with the specification
mandrel a month or more after they were laid, after which all openings
into splicing chambers were stopped by wooden plugs, 8 in. long tapering
from 3-3/4 in. at one end to 2-3/4 in. at the other end, and shaped to
fit the opening tightly. At first the plugs were paraffined, to keep
them from swelling and breaking the ducts, but were not successful, as
the paraffin lubricated them so that they would not stay in place. They
were expensive, and there was some swelling in the best that were
obtained. A better plug was made by using no paraffin, but by making six
saw cuts, three horizontal and three vertical, in the larger end,
cutting to within about 2 in. of the smaller end. The swelling of the
wood was then taken up by the saw cuts and the spring of the wood.

The splicing chambers are at 400-ft. intervals. They are 6 ft. long, 4
ft. 9 in. high, with a width varying from 3 ft. 2 in. at the top to 1
ft. 2 in. at the bottom.