The superiority of this instrument, over any other spirometer in use, rests in its simplicity of construction. It does not depend upon water which will evaporate, or metal which will corrode, as is the case with the Hutchinson spirometer; nor does it require a certain amount of resistance to be overcome, as in elastic measurers. With Marsh's rubber spirometer this resistance is found by actual measurement to be equivalent to twenty millimeters of mercury, before any test of the expired breath can be recorded.
This new spirometer is capable of giving in cubic inches of air,the exact measurement of the vital capacity of the most delicate child or woman or of the strongest man. It is large enough to contain 6 small or 2 large breaths, so that an average record may thus be obtained if desired. If rightly made it is as nearly perfect as such a simple thing