'224 Aitronomical Sociely. 



my attention particularly to the limb of the moon, allowing' tlie star 

 to approach, when it gradually disappeared behind the moon, until 

 one-half of its apparent disc was hidden, the last half disappearing 

 instantaneously. 1 also had an observation of the emersion on the 

 9th of December, when I allowed the star to pass from the moon for 

 about two seconds, and then directed myattention particularly to the 

 star. It appeared to stop, and then return towards the iiioon's limb." 



3. Observations made with a sextant of Dollond and a reflecting 

 circle of Troughton, for ascertaining within what degree of accuracy 

 these instruments would give the latitude of a known station, by 

 Capt. Basil Hall, R. N. 



At the request of Mr. Henderson, who was desirous of ascertain- 

 ing the degree of accuracy to be expected from nautical instruments. 

 Captain Hall undertook a series of observations to determine the la- 

 titude of the Calton Hill Observatory. The instruments used were, 

 a sextant by Dollond, of 11 inches radius divided to 10", the tele- 

 scope magnifying eighteen times; a reflecting circle by Troughton, 

 10 inches diameter, divided to 20", with three verniers ; and an arti- 

 ficial horizon by Dollond. The time was taken from a half seconds' 

 chronometer by Molyneux, compared before and after the observa- 

 tions with the transit clock. 



The latitude of the Calton Hill Observatory, as determined by 

 Mr. Henderson from the data of the Trigonometrical Survey, is 

 550 57'19",5. 



The computations were made by Mr. Henderson from Schuma- 

 cher's Hiilfstafeln, and no observation rejected. 

 dollond's sextant. 



The index always moved the same way in making the observation 

 and determining the index error. 

 1829, Latitude. 



Sept. 9, by a mean of 24 circum-meridional observations of j ^., c'_ ''„ » 



the sun ) ' 



16, by a mean of 16 (roof of horizon reversed) 55 57 18,1 



byameanof40 55 57 19,0 



troughton's reflecting circle. 

 Captain Hall conceives there is a slight degree of flexure in the 

 index bar ,• and that, if the index be pushed from the zero, face left, 

 and also from the zero, face right, the angles given by the vernier 

 will in both cases be too great, and versa vice*. To obviate this er- 

 ror, care was taken to move the index in the same direction, face 

 right and face left. 



Latitude. 

 Sept. 12, 13, 14, 15, 16, by a mean of 58 circum-meridional ) o / « 



observations of the sun, face east, \ 55 57 16,9 

 face west, the roof reversed, &c. ) 



• [If these precautions be neglected, it would seem, from Capt. Hall's 

 observations, that an extreme difference of l.'V might arise in his circle 

 from this cause alone. Query : Does not this difference arise from a twist 

 in the centre work ? — Sec] 



Captain 



