As the edges of the strips are slightly rounded,
they are filed away sufficiently to form good joints wherever the others
butt against them. The whole combination is bound together by ordinary
stove bolts, one quarter of an inch in diameter, placed near the center
of the width of the iron strips, and at a distance apart of about two
and one-half inches. Their heads are countersunk into the strap iron. In
making the holes for the stove bolts through the thin rubber, care
should be taken to make them sufficiently large to enable the bolt to
pass through without touching the rubber, otherwise the rubber may cling
to the bolts, and if they are turned in their holes the rubber may be
torn near the bolts and made to leak. A rough washer, under each nut,
prevents it from cutting into the back-board. For the purpose of
introducing air to, or removing air from, the pad, a three-eighths of an
inch lock nut nipple is introduced through the back-board, the
shellacked paper, and its thin paper covering. Without the back-board a
T connects with the nipple. One of its branches leads, by a rubber tube,
to the pressure gauge, which is a U-tube of glass containing mercury.
The other branch has upon it an ordinary plug cock, and, beyond this, a
rubber tube terminating in a glass mouth-piece. When it is desired to
inflate the air-cushion, it is only necessary to blow into the
mouth-piece.
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