244 Dr. Brinkley’s observations for determining 
And as far as my own observations are concerned, the diffe- 
rence 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 se- 
veral solstices are rather discordant, more so than was to be 
expected from a fixed instrument, yet it is likely a mean of 173 
observations cannot be far from the truth. 
This mean is 23 0 28' 1 i",o for 1769, 
when reduced to 1800, is 23 27 57 ,7, 
which agrees sufficiently near with the present determi- 
nation, 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 com- 
puted by M. Gerstner, gives for 1769— 23 0 28' 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 meri- 
dian, 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 facility of reversing the instrument 
seems more likely to produce exact results, than those ob- 
tained 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 discordant than those 
obtained by a fixed instrument. 
