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HYDROMETER. An instrument for measuring the density of fluids; being immersed in fluids, as in water, brine, beer, brandy, &c., it determines the proportion of their densities, or their specific gravities, and thence their qualities.
     2. By, the Act of Congress of January 12, 1825, 3 Story's' Laws U. S. 1976, the secretary of the treasury is authorized, under the direction of the president of the United States, to adopt and substitute such hydrometer as he may deem best calculated to promote the public interest, in lieu of that now prescribed by law, for the purpose of ascertaining the proof of liquors; and that after such adoption and substitution, the duties imposed by law upon distilled spirits shall be levied, collected and paid, according to the proof ascertained by any hydrometer so substituted and adopted.