10 



MR. RUMKER'S OBSERVATIONS 



RemarTts. — These results are somewhat contradictory to the last, the soutlerh 

 stars giving rather less for latitude than the northern, which wouldT prove that 

 the zenith distances had been observed too great. 



When a method leaves us in uncertainty, we must resort to another that is 

 independent of those errors that vitiated the former. One of the effects of 

 gravity is, that it causes eccentricity in the repeating circle when used in the 

 manner last described, by depressing the small circle, which carries the tube, 

 below the centre of the great circle's division ; so that the optical axes, or radii 

 of the double zenith distances, are removed downwards parallel to themselves, 

 and thus subtending a greater arc of the limb, make the observed zenith 

 distance too great. The observations alternately direct and by reflection, 

 are free of these errors, for the displaced vertex of the observed arc remains 

 in the diameter that is parallax to its chord. 



In the preceding observations, the refraction corresponding to the mean 

 height of the barometer and thermometer has been applied to the mean of the 

 zenith distances. There is no error in this, the change of the refraction being 

 in all tables assumed proportional to that of the barometer and thermometer 

 in so small limits. But I have also employed the true south polar distance 

 corresponding to the mean date, instead of correcting it for each particular 

 day. The error thence resulting is within the probable limits of the steadiness 

 of the level, from its being differently influenced by temperature on different 

 days. 



IV. Latitude by Observations alternately direct, and by Reflection from 



Mercury. 



1 . Observations of the Sun with the Mural Circle near the Southern 



Solstice, December 1827. 



1827. 



Barom. 



Therm, 



Observation. 



Microscopes. 



II. 



III. 



IV. 



Refr. 



Parall, 



L 

 L 

 L 

 U 

 L 

 L 



Dec. 25 



28 



Jan. 



1 



2 



4 



10 



inches. 



29.71 



30.03 

 30.05 

 30.044 

 30.114 



29.758 



102 

 83.3 



85 

 89 

 86 

 80. 



direct 



direct 



by reflection. 



by reflection. 



by reflection. 



direct 



67 4 31.6 



67 10 22.2 

 225 26 50.0 

 225 54 47.0 

 225 11 13.7 



68 22 38.9 



48.6 

 25.6 

 58.0 

 58.0 

 24.7 

 54.5 



46.5 

 18.3 

 7.5 

 1.0 

 30.3 

 49.8 



45.8 

 28.0 

 50.3 

 53.3 

 20.5 

 52.3 



9.53 

 10.19 

 10.4 



9.83 

 10.70 

 11.40 



1.6 



1.6 



1.65 



1.6 



1.7 



1.8 



i 



