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 slightly under 15 lbs. per square inch.

This fact is utilized in the pressure recorder shown in Fig. 167, a U-shaped glass tube half filled with mercury.  A rubber tube is attached to the bent-over end of one of the legs, so that the effects of blowing or suction may be communicated to the mercury in that leg.  Normally the mercury stands level in both tubes at what may be called the zero mark.  Any change of level in one leg is accompanied by an equal change in the opposite direction in the other.  Therefore, if by blowing the mercury is made to rise an inch in the left leg, the pressure exerted is obviously that required to support a two-inch column of mercury—­that is, 1 lb. per sq. inch.  This gives a very convenient standard of measurement, as every inch rise above the zero mark indicates 1 lb. of pressure.