488 
Mr. Ivory on the astronomical refractions. 
The errors of the French Table are + 340.8 — 119-1 
amounting to 459.9 when the signs are neglected ; and giving 
a mean error equal to 34 °- 8 — ll 9 l _ or 
n 44 
In the new Table we have + 218.6 — 196.3; the total 
sum being 414.9, and the mean error or half a 
second. 
In both views the advantage is in favour of the new Table. 
The inspection of the foregoing Table will show how fruit- 
less it would be to expect a near agreement in every single 
instance between observation and any table of refractions 
whatever. Thus, the zenith-distance is less, and the baro- 
meter and thermometer are both nearer the standard mean 
quantities on the 12th of March, than on the 5th of the same 
month, 1809 ; on all these accounts, the refraction should be 
less on the former day than on the latter ; whereas, according 
to observation, it is greater by n".5. There is, therefore, no 
sure test of the accuracy of a Table of Refractions except the 
smallness of the mean error in a series of observations made 
at different times. 
I shall now subjoin and compare with the new Table, ob- 
servations of a number of stars at low altitudes, for which I 
am indebted to the liberality of Stephen Groombridge, Esq. 
F. R. S. The reductions necessary for finding the true re- 
fractions were made by that astronomer ; and the practice of 
estimating the temperature of the air by the exterior ther- 
mometer, which he recommends as answering best with his 
method of observing, is followed in calculating from the 
