not unprofitable form of entertainment, as
even among adults these figures will be found to vary widely.

Air Volume Measuring.--The air which the lungs deal with is
scientifically classified under four heads:

1. Tidal air, which passes into and out of the lungs in natural breathing.
About 30 cubic inches in an adult (average).

2. Reserve air, which can be expelled after a normal expiration. About 100
cubic inches.

3. Complemental air, which can be drawn in after a normal inspiration.
About 100 cubic inches.

4. Residual air, which cannot be removed from the lungs under any
conditions by voluntary effort. About 120 cubic inches.

The first three added together give the vital capacity. This, as an
addition sum will show, is very much greater than the volume of air taken
in during a normal inspiration.

The simplest method of testing the capacity of an individual pair of lungs
is embodied in the apparatus shown in Figs. 165 and 166. A metal box is
submerged, bottom upwards, in a tank of somewhat larger dimensions, until
the water is level with the bottom inside and out. A counterweight is
attached to the smaller box to place it almost in equilibrium, so that if
air is blown into the box it will at once begin to rise.

If we make the container 7-1/16 inches square inside, in plan, every inch
it rises will represent approximately 50 cubic inches of air blown in; and
a height of 7 inches, by allowing for 325 cubic inches, with a minimum
immersion of half an inch, should suffice even for unusually capacious
lungs. The outside box need not be more than 8 inches all ways.

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

Unless you are an expert with the soldering iron, the making of the boxes
should be deputed to a professional tinman, who would turn out the pair for
quite a small charge. Specify very thin zinc for the air vessel, and have
the top edges stiffened so that they may remain straight.

On receiving the boxes, cut a hole 3/4-inch diameter in the centre of the
bottom of the air vessel, and solder round it a piece of tubing, A, 1 inch
long, on the outside of the box. In the centre of the larger box make a
hole large enough to take a tube, E, with an internal diameter of 1/8 inch.
This tube is 8 inches long and must be quite straight. Next procure a
straight wire, C, that fits the inside of the small tube easily; make an
eye at the end, and cut off about 9 inches. Bore a hole for the wire in a