As the unit of length was also lost, a series of experiments was made in the vibration of a pendulum in a vacuum, marking seconds of mean time in the latitude of London at the level of the sea. These deductions, however, failed to be satisfactory, and the commission was compelled to fall back upon the best preserved of the duplicate standards existent. The Imperial Standard Pound is declared to be the true weight of an avoirdupois pound in a vacuum. It is a curious fact that the Imperial standards of platinum (which metal is not subject to oxidation), although balancing brass weights in a vacuum, weigh in air more than one-half a grain heavier than the latter. This is due to their greater displacement of space. The unit of weight in the United States is a Troy pound weight obtained from England, a duplicate of the original standard fixed by the commission of 1758, and reasserted by the commission of 1838. It is a bronze weight of 5,760 grains Troy. It is kept in a strong safe at the United States Mint, in Philadelphia. The President appoints an assay commission, whose members meet at Philadelphia annually, upon the second Wednesday in February, open the safe, and compare the copies, or the working weights, with the original upon the most delicately poised balances. Working standards of weights and measures are supplied by the Secretary of State to the State governments, which in turn supply them to the sealers of weights and measures of the various countries, who must compare with the State standard once a year. [Illustration: TROY STANDARD POUND WEIGHT. Fac-simile, exact size.] All of the scales and delicate test instruments in use by the government, not only in Philadelphia Mint, but at the several branch mints, are manufactured in this country, and as examples of wonderful mechanical machines of minute accuracy they lead the world. Some of them are the work of Mr. Henry Troemner, of Philadelphia, to whom, it is proper to say, the writer is largely indebted for the facts given in this article. Mr. Troemner, in the capacity of government expert, makes frequent visits to the most distant points in the Union for the verification of national standards. The Treasury Department made an especial request of him to exhibit at the New Orleans Exposition, a line of his fine balances. EXTRACT FROM CONSTITUTION OF THE UNITED STATES. ARTICLE I., Sect. 8. The Congress shall have power ... to coin money, regulate the value thereof and of foreign coins, and fix the standard of weights and measures, ... to provide for the punishment of counterfeiting the securities and current coin of the United States. ARTICLE I. Sect. 2. No State shall ... coin money, emit bills of credit, make anything but gold and silver coin a tender in payment of debts, ... Coinage, fiscal year 1887.