214 Observations for delermining 



The mean of IS observations near the winter solstice gives me 

 mean obliquity Jan. 1, 1813, 23" 27' 'JS",14. 



The above determination of the obliquity by observations 

 near the summer solstice gives (taking the annual diminution 

 0",43), 



Mean'obliquity Jan. 1, 1800 = 23° 27' 56",0, differing only V 

 from that assumed in M. Delambre's tables of the sun. And as 

 far as my own observations are concerned, the difference does not 

 exceed half a second. 



In M.Zach's solar tables, there is given a determination of the 

 obliquity of the ecliptic computed by M. Gerstner, from a mean 

 of a great many observations of Dr. Maskelyne's, made at 19 

 summer solstices. Although the results of the several solstices 

 are rather discordant, more so than was to be expected from a 

 fixed instrument, yet it is likely a mean of 173 observations can- 

 not be far from the truth. 



This mean is 23° 28' 1 1",0 for 1769, 



when reduced to 1800, is 23 27 57, 7, 

 which agrees sufficiently near with the present determination, to 

 show that, if the necessary corrections for the sun's latitude, &;c. 

 had been used, the result would probably have been very exact. 



The mean of 102 observations at 17 winter solstices computed 

 by M. Gerstner, gives for 1769 — 23" 2S' 3'' ; a result which, 

 after making all possible allowances for the error of the quadrant, 

 is considerably less than that deduced from the summer solstices. 



In using the eight feet circle, two or more observations were 

 made a few minutes before the sun arrived at the meridian, and 

 then the instrument was reversed, and observations made after 

 the passage. The results were carefully reduced to the meridian; 

 the upper and lower limbs being observed, the zenith distance 

 of the centre was deduced from the instrument itself. This fa- 

 cility of reversing the instrument seems more likely to produce 

 exact results, than those obtained by a fixed instrument, although 

 from the necessary effect of the action of the sun on the parts of 

 the instrument, the results must be expected to be more discord* 

 ant than those obtained bv a fixed instrument. 



The 



