266 Principles ofjijiding the Longitude. 



coincide in one plane, those meridians will in the same time re- 

 move — the earth's to the east, the stars to the west of the meri- 

 dian previously opposite — in such sort as that the t/ie7i and now 

 opposed meridians of each sphere will be an equal number of 

 their respective degrees apart. 



The duration of the earth's revolution around its axis is, in 

 mean solar time, Hours 23 56' 4" 7'" 13'^'^' 13^'^^ 36>"h= ; 

 which lapse of TIME may be measured by an index orace tracing the 

 periphery of a dial circle, graduated, and also divided into parts 

 of 10° figured 1 to 36, to assist the eye whilst inspecting the 

 quantity of an arc. 



By this MEASURER of time in accord with the relative motions 

 of the fixed stars and the earth, let the instants of the culmina- 

 tions of conspicuous stars be successively noted in the manner 

 of a table; — with this measurer and the table, an observer, 

 wherever situate on the globe, can quickly ascertain how much 

 westward or eastward of the first meridian our meridian is ; and 

 whether it bears east or west from the first. For the index, the 

 stars, the globe really, all move in equable and known revolution, 

 i. e. the index, any star but those which are precisely vertical at 

 the poles, — any place on the globe's surface but the poles, — all 

 pass through 360 degrees in the same time, and consequently 

 through equal arcs of their respective circles of revolution in the 

 same portion of that time ; wherefore, regarding what time one 

 of those conspicuous stars culminates to the place for which the 

 longitude is required, we have at once the information sought ; 

 the arc contained between the part of the dial circle to which 

 the index of the measurer at that instant points, and that to 

 which it points when the star culminates at the first meridian 

 (known by the table), being equal to the longituditial arc on 

 the globe between the Jirst meridian and the meridian of the 

 place ; and the place is west or east, according as the present 

 instant is in the succession of the parts of time nearer to the 

 moment when the star last culminated at the first meridian, or 

 to the moment when it will culminate there next. 



No objection on account of the variability of chronometers 

 from change of climate, or other causes, can hold good against 

 the means here proposed, that will not at the same time impugn 

 the methods now in use. It is to be recollected also, that the 

 high degree of accuracy to which time-keepers can now be 

 brought, is probably but a trifle less than the utmost precision 

 which the constitution of the materials best adapted for this pur- 

 pose will allow the art of man to gain. 



Further, the advantage of the method now advanced, in leaving 

 to the mariner a short and plain practice, is very considerable; 



for 



