Mr. Ivory on the astronomical refractions. 
477 
stances, it will be more compendious to compare it with 
other tables that have been long in the hands of astronomers, 
and the characters of which are well established. The table 
has been constructed with the same elementary quantities as 
the French table, at least as far as regards the refractive 
power and the weight of the air, which quantities alone in- 
fluence the magnitude of the refractions near the zenith. 
But, in comparing the two tables, an allowance must be 
made for the difference of the standard barometers ; and this 
requires that the refractions in the new table be all diminished 
by = — . Now, taking the refractions at 45 0 and 8o° 
from the zenith, we get 
and, in the French table, we find the first of these numbers 
exactly, and 319" .8 in place of the second. But we must not 
forget that there is a small subtractive correction to be ap- 
plied to the mean refractions of the French astronomers, 
which is usually neglected, although it will be found among 
the tables of refraction (Table V.) inserted in the Tables 
Astronomiques , published by them in 1806. This correction 
amounts to o".5 at 8o° from the zenith ; and the former 
number is thus reduced to 3 19". 3, the same as in the new 
table. We may therefore conclude, that when we calcu- 
late rigorously, the mean refractions of the new table are 
the same as those of the French astronomers, as far as 8i° 
or 82° from the zenith. 
But at lower altitudes there will no longer be the same 
