n. Series of selected glass tubes of constant, standard diameter, which is filled with liquids of various viscosities, except for a small air space at the top. When these tubes are inverted, the air bubble travels through the liquid, and the rate of travel is a measure of the viscosity of the liquid. Empty tubes of similar standard dimensions are filled with liquids of which the viscosity is required, and the rates of travel of the bubbles compared with those of the tubes containing the liquids of known viscosities. Viscosities are compared under controlled temperature conditions (Paul N. Gardner Co. Inc., 316 N. E. Fifth Street, Pompano Beach, FL, www.gardco.com).