March 28, 1890.] 



SCIENCE. 



207 



— The Scribners will publish immeiliately a small book by 

 George W. Cable, entitled "The Negro Question," containing the 

 address delivered by the author on Washington's Birthday before 

 the Massachusetts Club in reply to the memorable speech by the 

 late Henry W. Grady ; also several open letters by Mr. Cable on 

 this subject, 



— The Scribners will issue shortly the first of a series of inter- 

 esting memoirs of "Three Famous Fi-ench Women," translated 

 from the French of M, Imbert de Saint-Amand, by T. S, Perry. 

 The subject of the first volume will be the Empress Josephine, 

 and will be entitUd "The Wile of the First Consul," Other 

 works will follow on Marie Antoinette and (he Empress Marie 

 Louise. 



LETTERS TO THE EDITOR, 



Means of increasing the Accuracy of locating Vessels at Sea, 



In looking over some meagre accounts of the recent meeting 

 of the International Maritime Congress, I failed to find any 

 mention of a very important branch of navigation to which 

 my attention was attracted some years ago, I refer to the 

 question of the present accuracy of the instruments for deter- 

 mining the position of a ship at sea, and the steps that must be 

 taken in order to improve this accuracy. The reasons that make 

 this an important matter are so obvious that it is not neces- 

 sary to catalogue them. I need only say that in approaching 

 any coasts, dangerous reefs, shallow waters, and, in the case 

 of sailing-vessels, the paths regularly frequented, by steamers, 

 it is of the greatest importance to be able to locate the ship's 

 position with all the accuracy attainable. 



In 1881 Profefsor W. A. Rogers, the eminent American as- 

 tronomer and physicist, read a paper before the Naval Institute 

 at Annapolis, published in the "Proceedings" of the institute, 

 bearing the title "The Co-efHcient of Safety in Navigation." 



This paper is spoken of by Commander P. F. Harrington,^ 

 U.S.N. , as being "remarkable for the extent and thoroughness 

 of its investigations, and valuable in the application of its re- 

 sults to the practice of navigation. , . . Its ijractical conclu- 

 sion and warning ought to be impressed upon every man who is 

 permitted to lay a vessel course," 



In this paper Professor Rogers shows the various errors which 

 enter into the determination of a ship's location at sea; and 

 he determines, finally, an average error and a possible error of 

 position from a large number of observations, as shown by logs 

 of vessels in various quarters of the globe. Upon his inquir- 

 ing of a number of sea-captains as to the limits within which 

 a ship's place can be ordinarily determined, most of them said 

 a mile was the limit, some few said half a mile, and only one 

 man gave so high an estimate as five miles. 



The chief sources of error seem to be those pertaining to the 

 compass, chronometer, and sextant. For the compass. Professor 

 Rogers does not come to any very definite conclusion; at least, 

 no numerical estimates of error are made, 



A discussion of the rates of a large number of chronometers 

 shows, that, for a chronometer of average excellence, at the end 

 of twenty days an average error of 3.6 miles must be expected, 

 and an error of 11,5 miles must be looked out for. The error 

 of the chronometer increases with the time occupied in the voy- 

 age; and a discussion of the errors of one hundred chronometers 

 by Mr. Hartnup of Liverpool (and probably no more capable 

 man has ever examined into the matter) showed that at the 

 end of a voyage of twelve months the error in one of the ship's 

 positions was .524 miles. Another extreme instance cited is 

 the case of Lord Anson's voyage around Cape Horn, in which 

 one ship "actually made land on the wrong side of the con- 

 tinent, the error of position being over 600 miles." 



For the sextant observations it is difficult to determine the 

 limit of accuracy; but "the average error of a single observa- 

 tion at sea is not far from 3 miles, and the average co-efiScient 



ed at Editor's Offic 



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