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The capacity of the lungs, and their powers of inspiration and expiration, can be tested by means of easily constructed apparatus which will interest most people who are introduced to it. The reduction of the capabilities of the lungs to figures affords a 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: Tidal air, which passes into and out of the lungs in natural breathing. About 30 cubic inches in an adult (average). Reserve air, which can be expelled after a normal expiration. About 100 cubic inches. Complemental air, which can be drawn in after a normal inspiration. About 100 cubic inches. 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.