692 THE POPULAR SCIENCE MONTHLY. 



cal 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 sci- 

 entific 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 pe- 

 riod were his Tidal Eesearches, 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 18G0, however, it was found that the 

 position of the moon was somewhat in advance of its calculated 

 position ; all the known efi'ects 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 presenta- 

 tion 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 deter- 

 mined 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 stud- 



