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.
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