Mr. Galbraith on the Obliquity of the Ecliptic. 15 



is in its mean state, or the barometer at 30 inches, and Fah- 

 renheit's thermometer at 50°, in which case at 75°, Z. D., the 



i a 

 French tables will give the refraction = 3 34*90 



Bessel's 3 34*30 



Brinkley's 3 34-80 



Young's (like Bessel's) 3 34-30 



Ivory's 3 34-70 



Mean of the whole 3 34-60 



Bradley's 3 33-04 



Probable error of Bradley's — 1-56 



That is, the correction to be added to the zenith distance is 

 too small by l"-56 in winter, and consequently the obliquity 

 is also too small by the same quantity, on account of the error 

 in Bradley's Refractions. This would, however, be partly 

 corrected by the proportional defect at the summer solstice, 

 which amounts to — 0"-34 ; consequently the whole difference 

 arising from this cause would be the difference between l"-56 

 and 0"-34, or l"-22 to be added to 3"-00 formerly obtained, 

 making 4"-22 on the whole. 



It may therefore, I think, be reasonably concluded, that 

 when the latitude of Greenwich is assumed at 51° 28' 40", and 

 the observed zenith distances of the sun at the solstices is cor- 

 rected by Bradley's table of Refractions, the difference between 

 the summer and winter obliquity will, independent of errors 

 of observation, amount generally to 4"*22. 



Now it will appear from an examination of the Greenwich 

 determinations from about the year 1765 till 1790, in Dr. Pear- 

 son's table formerly alluded to, that the mean difference du- 

 ring that period is very nearly equal to that which has now 

 been found. From 1790 till 1811 the differences are so great 

 (about 10" or 12") that there must of necessity be some other 

 error mixed up along with it. Perhaps the error of the mural 

 quadrant then used was the principal cause. 



Mr. Pond's observations from 1812 till 1819 inclusive, give 

 a mean of — 4"*87, so nearly the same as — 4"*22, the quan- 

 tity obtained on our theory, as to be a strong confirmation of 

 its justness. 



It is also confirmed by the observations of M. Bessel, 

 whose results may be seen in table 4, page 436, of Dr. Pear- 

 son's work already referred to. 



I am aware, however, that M. Cacciatore, superintendent 

 of the Observatory of Palermo, has advanced a theory of this 

 discrepancy of the obliquity, as determined by observations at 

 the summer and winter solstices, depending on the effects of 



the 



