termination between the observatories at Paris and Greenwich, which supersedes the value previously admitted, correcting it by nearly half a second of time. His essay on "Tides and Tidal Action in Harbors," first published as a lecture before the American Institute, is remarkable for its lucid and terse exposition of principles without the aid of mathematical symbols. While possessing great facility in employing the aid of the higher mathematics, Mr. Hilgard systematically avoids, as far as practicable, their introduction in his writings, preferring to use logical statements of the processes of reasoning.
As part of the duties of his office, Mr. Hilgard has charge of the construction and verification of the standards of weight and measure for the United States, and, by order of Congress, has been for some years past engaged in preparing metrical standards of great precision for distribution to the several States. In this connection he was appointed a delegate to the International Metrical Commission, which met at Paris in 1872, having for its object the construction of new metrical prototypes of great precision and permanence, and which has since resulted in the establishment of an International Bureau of Weights and Measures at Paris, under the direction of a committee, of which Mr. Hilgard is a member. A valuable and instructive treatise on "Methods of Precision in measuring and weighing" was read by him before the Stevens Institute of Technology, but has not yet been published.
When, in 1863, the National Academy of Sciences was chartered by Congress, Mr. Hilgard was one of the original members named in the act. He is at present the home secretary of that scientific body. The compliment of honorary membership has been conferred upon him by the American Philosophical Society of Philadelphia, and the Academy of Arts and Sciences of Boston. His frequent communications to the Philosophical Society of Washington are evidence of a very active interest in scientific research, maintained notwithstanding the exactions of his arduous official labors. A work of great interest, which he is now conducting outside of his official sphere, is a magnetic survey of the United States, prosecuted at the expense of the Bache Fund, arising from a bequest of the late Alexander Dallas Bache to the National Academy of Sciences.
No small part of Mr. Hilgard's services to science and education is to be found in the readiness and obliging disposition with which he has constantly given information and rendered facilities by the loan of instruments and apparatus to persons engaged in scientific research or instruction. Besides meeting numerous requests of this kind at home, he has given his best aid and advice to the equipment of government surveys in the Sandwich Islands and in Japan. Although Mr. Hilgard's scientific work has been generally limited to the sphere embraced in his practical pursuits, he has been a very active student in other branches of science, especially dynamics and molecular physics.