keyboard, for not only do pipes of different shapes give different
qualities of tone, but it is found desirable to have ranks of pipes with
their bottom note of different pitches. The length of an open pipe is
measured from the edge of the lip to the top of the pipe; of a stopped
pipe, from the lip to the top and back again. When we speak of a 16 or 8
foot rank, or stop, we mean one of which the lowest note in the rank is
that produced by a 16 or 8 foot open pipe, or their stopped equivalents
(8 or 4 foot). In a big organ we find 32, 16, 8, 4, and 2 foot stops,
and some of these repeated a number of times in pipes of different shape
and construction.

THE ARRANGEMENT OF THE PIPES.

We will now study briefly the mechanism of a very simple single-keyboard
organ, with five ranks of pipes, or stops.

[Illustration: FIG. 139.--The table of a sound-board.]

It is necessary to arrange matters so that the pressing down of one key
may make all five of the pipes belonging to it speak, or only four,
three, two, or one, as we may desire. The pipes are mounted in rows on a
_sound-board_, which is built up in several layers. At the top is the
_upper board_; below it come the _sliders_, one for each stop; and
underneath that the _table_. In Fig. 139 we see part of the table from
below. Across the under side are fastened parallel bars with spaces
(shown black) left between them. Two other bars are fastened across the
ends, so that each groove is enclosed by wood at the top and on all
sides. The under side of the table has sheets of leather glued or
otherwise attached to it in such a manner that no air can leak from one
groove to the next. Upper board, sliders, and table are pierced with
rows of holes, to permit the passage of wind from the grooves to the
pipes. The grooves under the big pipes are wider than those under the
small pipes, as they have to pass more air. The bars between the grooves
also vary in width according to the weight of the pipes which they have
to carry. The sliders can be moved in and out a short distance in the
direction of the axis of the rows of pipes. There is one slider under
each row. When a slider is in, the holes in it do not correspond with
those in the table and upper board, so that no wind can get from the
grooves to the rank over that particular slider. Fig. 140 shows the
manner in which the sliders are operated by the little knobs (also
called stops) projecting from the casing of the organ withi