5Q On the Elements for reducing the Polar Distances 



meridiona! zenith distances of y Draconis, observed at the 

 same tinu- both witli the sector and quadrants. The zenith 

 sector used by L'.eut. -colonel Wiidge in the trigonometrical 

 survfv is divided on an arc of eight feet radius. And 

 from the credited ability of the maker of this latter instru- 

 iTient, and the ]>ro2;ressive state of improvement in the con- 

 struction of astronomical instruments at the period it was 

 finished, a much greater degree of consistency might have 

 been expected, than what is found amongst the deduc- 

 tions of the observations made with it, that have been al- 

 ready pubhshed. The number of observations made at 

 each station, however, nuist tend to render the final result 

 almost free from error, and the anjplitude of the celestial 

 arc as correct as perhaps it can be obtained. The observa- 

 tions made with this instrument lor determining the length 

 of a de-'ree of the arc of the meridian have tended to show 

 that the plumb-line with which the instrument was adjusted, 

 is deflected from the perpendicular, from local attractions 

 upon the earth's surface, and that consequently the ob- 

 served measure of the celestial r.rc contained between the 

 zenith of two places, is not their true difierencc in latitude. 

 Over a small country like that of England, the difference of 

 latitudes of places uiiiy be determined with almost any de- 

 (Tree of accuracy by a tiigonometricaj survey ; and it is pre- 

 sumed that the general survey which has been carrying on 

 for several years past, under the direction of the Ordnance, 

 has not been deficient in this respect, as the persons to 

 whom the operations have been intrusted are provided 

 with the best inFtrumeiits perhaps that frtmi the state of 

 modern improvenients could have been contrived for the 

 purpo-iC. Althougli genilh sectors are undoubtedly the 

 most accuratp u'lslruments for determining zenith di- 

 stances, a very considerable degree of aceuiacy may be 

 obtauied by small instruments of an appropriate construc- 

 tion, as nieridian circles, repeating circles, and even the 

 circle by reflection wiien properly mounted upon a stand, 

 the inaccuracy beii.g continually lessened by a succession 

 of observa.ions either upon the same star, or upon a number 

 of sta»,;^ so as at last to become only the fraction of a secon;]. 

 If a series of astronomical observations on the dislauees 

 of several stars from the zenith were accurately made at 

 difl'erent stations, or at such objects as were bisected in the 

 survey, and whose situations have been correctly reduced, 

 results of considerable interest might be deduced, as the de- 

 gree, of reflection of the plumb-line at each station might 

 be determined. 



In 



