AT THE OBSERVATORY AT PARAMATTA. 



38 



Methods of Observing the Repetitions. 



During the last years when I was without an assistant, the intervals, and 

 therewith the second parts of the reductions p^ cot z {s — I) would have become 

 too great in the southern solstice, if I had attended at the same time to the 

 level, which moreover became useless under the sun's vertical rays. This is 

 therefore an additional reason why I have resorted to reflection from mercury. 

 The small nadir distance enabling me to place it upon the same isolated pillar 

 with the instrument, and to keep all the openings of the dome shut except the 

 top slide, the mercury was secured against wind, and all percussion save that 

 occasioned by handling the instrument, and no glass cover was required. During 

 the same series I did not revolve 180° in azimuth, but pointed the tube in a left- 

 sided series, first by means of the great circle to the reflected image, and next 

 by shifting the small circle to the direct object, and I then again turned the 

 great circle for the observation by reflection, repeating this process until a 

 series was completed, containing a multiple of altitudes instead of zenith di- 

 stances. In a right-sided series it is the great circle by which the tube must be 

 first pointed to the direct object. The repetitions can thus be carried on with 

 remarkable expedition by one observer. All that is required is that the in- 

 strument remains steady during the interval between a reflected and direct 

 vision. The reflection and direct vision enabled me by three observations to 

 verify that the optical axis described a vertical circle, and the effects of bending 

 compensated themselves. I shall illustrate the process by an 



Example. 



December 22, 1827 : Barom. 29.720. Therm. 83°. Transit per Clock 17'' SS"* 1'.9. 



f2 



