Lieut. Davis on the Prime Meridian. 399 



the French, Portuguese, and Dutch, have placed their prime me- 

 ridians out of their own country; but I need not consume space 

 by entering into historical details that are easily obtained.* 

 The early conduct of the French, however, in this matter, de- 

 serves to be mentioned because it contains some instructions 

 for ourselves. By a royal ordinance of Louis XIII, the island 

 of Ferro was established as the French first meridian, and Paris 

 was assumed to be twenty degrees to the eastward of it. It 

 appears from the memoirs of the Royal Academy of Sciences 

 of 1742 and 1746, that attempts were subsequently made to fix 

 with accuracy the exact distance of Paris from the island, not- 

 withstanding that no precise spot on it had been designated as 

 the origin of longitudes, and that there was a prevailing igno- 

 rance as to the topography and shape of the island. The deter- 

 minations by different persons of course varied, and this caused 

 those uncertainties and fluctuations in the French longitudes 

 which led to the final abandonment of the assumed meridian. 

 We may profit by this example. 



Having decided to take for our prime meridian that great circle 

 of the earth which is ninety degrees or six hours from Green- 

 wich, we are to keep it wherever it may fall. By means of the 

 magnetic telegraph, the distance of this circle from the meridian 

 of the National Observatory can be determined with sufficient 

 accuracy, and, being once determined, it is to be regarded as fixed 

 and permanent. If it should be found necessary to make any 

 change hereafter, that change will be applied to the imaginary 

 meridian, and not to the meridional differences of other places 

 from Washington, which are to remain always the same. 



The Washington Observatory is thus made the virtual standard 

 according to which all values are assigned, and to which all me- 

 ridional differences are referred; and from which, also, all abso- 

 lute longitudes are computed. 



Its own distance from the six hour circle being once ascertained 

 by magnetic communication, it will be, in effect, for this country, 

 the true origin of longitude. 



It will not be practically indispensable to distinguish by any 

 visible, real mark, the meridian of New Orleans, so far as the 

 National Observatory is concerned ; for the latter, its distance in 

 time from the arbitrary meridian being once assumed, becomes 

 the effectual, established zero; but this mark will be useful for 

 reference in the adjacent country, saving labor and time in fixing 

 longitudes in its vicinity, and its foundation appears to be pecu - 

 iarly p rop er as a national monument. The cost of such a mark 



^ill be but trifling. . ,. 



Thus the new meridian will be, what its name implies, strictly 

 arbitrary. It may be thought that there a re reasons of a scien- 



art. Geog.— Good's Pantologia, and Lon- 



art. Meridkn— Delambre's Hist, of Astronomy, Mackay on Long., <fcc. 



