

216 Application of the Galvanic Circuit 



reversed position of the instrument are imprinted, the levels are 

 again read, and the registering fillet labeled. The same operation 

 is repeated on the same star at the western station. The electro- 

 magnetic clock may be rated at the central station. The auto- 

 matic register may be kept there, and the printed record may be 

 there preserved. Check registers may be kept at the other sta- 

 tions. The imprinted culminations of two fundamental stars at 

 the same station, whether eastern, or western, or central, will 

 serve to fix the rate of the primitive clock. The reversals re- 

 move the error of collimation, and furnish, by taking the means 

 of the printed dates for each wire, twenty normal bisections 

 with reference to the optical axis of the telescope. On apply- 

 ing the corrections for the levels, we have twelve normal level 

 readings with reference to the zenith and consequent meridional 

 point near the position of actual observation. The facility of ob- 

 serving transits of circumpolar stars, gives extraordinary precision 

 to the azimuth correction, which moreover nearly vanishes with 

 circumzenith stars. 



A consideration of these numerous advantages of the print in 

 method for longitude of stations, leads me to believe that one 

 single culmination of a circumzenith star at each of the two sta- 

 tions thus observed, may give their relative longitudes to a few 

 hundredths of a second, which is all that can be said of our pres- 

 ent knowledge of those of the Greenwich and Paris observatories, 

 the two oldest in the world. The check readings of the levels 

 with nadir pointings adds to the certainty of the work. If the 

 concluded difference of longitudes of the zenith and nadir por- 

 tions of the meridians conform, our confidence in the result is 

 much strengthened. 



If the discrepancy is sensible, the mean of the results may serve 

 to eliminate any want of symmetry in the pivots or sockets, or any 

 irregular action of gravity or sudden change of temperature on 

 the parts of the instrument. 



In the ibove process all sources of error in the result are elim- 

 inated except the difference of the break circuit armature times, 

 and personal equations of the astronomers in tapping on the key. 

 The rule adopted in good telegraphic offices, and which must be 

 inflexibly adhered to in all astronomical operations, of adjusting 

 the outer and inner limits of armature distance to a mmimum 

 value, secures us against error from the first source. When the 

 total quantities are so small as to be insensible to the ear, their 

 difference, which is a term of the second order, must necessarily 

 be evanescent. 



The personal equations of tapping on the key for the instant 

 of a bisection are much smaller than those which arise from the 

 combination of sight and hearing. 



