From: Pneutherapy including aerotherapy and inhalation methods and therapy. By Paul Louis Alexandre Tissier. 1903, page 25. “A U-shaped tube, open to the atmosphere and suitably mounted, is filled with mercury in both branches to the same level which is marked zero. One branch is connected with a rubber tube and mouthpiece (or mask or nosepiece) used by the person under observation, whose expiratory and inspiratory force is measured by the ascent or descent in the mercury in the other branch, as shown on a millimetric scale.”