metal disc 1 inch across.

[Illustration: FIG. 166.--Perspective view of lung-capacity tester.]

The air container is then placed in the water box and centred by means of
wooden wedges driven in lightly at the corners. Push the small tube through
its hole in the water box, and thrust the wire--after passing it through
the disc and the projection on the air container--into the tube. The tube
should reach nearly to the top of the air container, and the wire to the
bottom of the water box. Solder the tube to the box, the wire to the disc,
and the disc to the container. A little stay, S, will render the tube less
liable to bend the bottom of the box. Plug the tube at the bottom.

The wire sliding in the tube will counteract any tendency of the container
to tilt over as it rises.

A nozzle, D, for the air tube is soldered into the side of A, as shown.

The counterweight is attached to the container by a piece of fine strong
twine which passes over two pulleys, mounted on a crossbar of a frame
screwed to the sides of the water box, or to an independent base. The
bottom of the central pulley should be eight inches above the top of the
container, when that is in its lowest position.

For recording purposes, make a scale of inches and tenths, and the
corresponding volumes of air, on the side of the upright next the
counterweight. The wire, W, is arranged between counterweight and upright
so that an easily sliding plate, P, may be pushed down it by the weight, to
act as index.

[Illustration: FIG. 167.--Apparatus for showing lung power.]

Notes.--The pulleys must work easily, to reduce friction, which renders
the readings inaccurate. Absolute accuracy is not obtainable by this
apparatus, as the rising of the container lowers the water level slightly,
and the air has to support part of the weight of the container which was
previously borne by the water. But the inaccuracy is so small as to be
practically negligible.

A Pressure Recorder.

[Transcribers note: Even with the precautions used in this project, health
standards of 2004 would consider any exposure to mercury dangerous. Water
could be substituted and the column lengths scaled up by about 13.5.]

If mercury is poured into a vertical tube closed at the bottom, a pressure
is exerted on the bottom in the proportion of approximately one pound per
square inch for every two inches depth of mercury. Thus, if the column is
30 inches high the bottom pressure is sligh