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692 Almanac, then prepared in Cambridge, Mass., and thus opened the happier situation of his later years; but it was not until the spring of 1858 that he finally left Nashville. From this time on he did not lack opportunity for study and acquaintance with scientific men. In 1867 he joined the Coast Survey, then under the superintendence of Prof. Benjamin Peirce, and remained in that service until 1883. The chief results of his work during this period were his Tidal Researches, Meteorological Researches, and his Tide-predicting Machine, all of which contribute to his well earned reputation.

Ferrel's researches on the tides were in both theoretical and practical directions. His theoretical discussions began in his days of teaching in Kentucky, and in 1853 had led him to conclude that the action of the tides would very slightly retard the rotation of the earth, but at that time no indication of such retardation had been found by astronomers. In 1860, however, it was found that the position of the moon was somewhat in advance of its calculated position; all the known effects of external perturbations having been allowed for, its advance still was unexplained. Ferrel, then living in Cambridge, returned to this problem and showed that the moon's unexplained advance might be accounted for as only an apparent result, the real fact being a retardation of the earth's rotation by tidal action. The essay on this subject was published in the Proceedings of the American Academy of Arts and Sciences in Boston in 1864. An incident in this connection illustrates the diffidence that Ferrel felt in coming in contact with strangers. He carried his essay on The Influence of the Tides in causing an Apparent Secular Acceleration of the Moon's Mean Motion in manuscript to the meetings of the Academy time after time, with the intention of reading it, but his courage always failed, until at last the paper was presented in 1864. Had its presentation been deferred over one more meeting, its appearance would not have antedated a similar essay by the French astronomer, Delaunay, on the same subject.

This was before Ferrel was a member of the Coast Survey; it was naturally followed by his engagement as expert in tidal studies in that office; and when afterward in Washington, he discussed and reduced many tidal observations made at various points on our coast. To lighten the labor of such computations he invented a tide-predicting machine, by means of which the time and value of high and low tides can be mechanically determined for various ports with sufficient accuracy for publication in the official tables, after the constants for the ports are worked out. This machine is now in regular use in Washington, where it is regarded as doing the work of thirty or forty computers. A general work on tides and their theory was among the latest