188 



The Variation of the Needle. 



Telegraphic longitudes, and time determinations between any two 

 stations are liable to peculiar and uncertain local errors — chief among 

 which is the disleveling of the astronomical instruments at each ter- 

 minus caused by the local differences in the direction of the theoretical 

 vertical from (he actual lines of gravitation, so that time stars are not 

 observed similarly (that is, under similar conditions), while the result- 

 ant deflection of the horizontal plate of the transit instrument from the 

 symmetric geodesic horizon occasions uncertain deviations in azimuth 

 Avhich cannot be corrected when the observations are not tied together 

 by accurate triangulation — the only precise method of li ic i e 3 

 of great distances. 



On this account, therefore, surveyors determine their meridian lines 

 by observations in the field, and not by deduction from the differences 

 of longitude between some observatory and the extremities of the base- 



The longitudes of stations are determined after much careful and 

 difficult work, and adjusted by complicated computations, but the 

 local azimuths of the triangle sides, and the local meridian lines deter- 

 mined in the field are essential in the adjustment of the longitude of 

 base-line termini, and the local meridian is the line from which each 

 declination or variation of the magnetic needle is measured. 



It is not here necessary to describe the methods employed in geo- 

 desic work in locating the prime meridian of a triangulation. 



With the larger forms of the portable transit, with the alt-azimuth 

 instrument, and the twenty and twelve-inch theodolites, the direction 

 of the true meridian is determined with the exactness requisite for the 

 computation of the geodesic azimuths, latitudes and longitudes. By 

 the observations of stars with the telescopes of high power, with which 

 these large instruments are provided, the astronomical co-ordinates of 

 each geographical position are determined with a high degree of pre- 

 cision, and at such stations it is merely necessary to place the zero 

 point of the magnetic instruments, used, in the true meridian deter- 

 mined, and the differences then observed in the direction of the 

 magnetic needle are the variations sought — the declination being the 

 pointing of the needle to eastward or westward of the true meridian — 

 reduced to the instant of diurnal magnetic quietude. 



The primary meridians of a great survey are often twenty miles apart, 

 and in order to obtain the true direction of the boundary lines in the 

 dense forest, intermediate between the mountain peaks, and to trace 



