448 Mr. Ivory on the astronomical refractions, 
the Table of Refractions published in the Connaissance des Terns , 
with which it is proposed to compare it. The comparison is 
contained in the following: Table: 
O 
Zen. Dist. 
Formula. 
Con. Des. T. 
Diff. 
O 
/ n 
/ II 
B 
45 
0 58.2 
58.2 
O 
60 
1 40.6 
1 40.6 
O 
70 
2 38.8 
2 38.8 
O 
80 
5 1 9.3 
5 19-8 
0-5 
85 
9 5 1.7 
9 54-3 
2.6 
86 
11 44.2 
11 48.3 
4.1 
87 
14 21.5 
14 28.1 
6.6 
88 
18 11.9 
18 22.2 
10.3 
89 
24 8.6 
24 21.2 
12.6 
90 
33 54-3 
33 46.3 
— 8.0 
The formula agrees exactly with the table till 8o° of zenith 
distance, when the difference is o".g. But if we turn to the 
Tables Astronomiques , published in 1806, by the French Board 
of Longitude, we shall find that there is a small correction to 
be subtracted from the mean refractions ; and when this is 
taken into account, the perfect agreement between the for- 
mula and the table will be restored. In like manner there 
are subtractive corrections to be applied at all other zenith 
distances ; and these increase very swiftly in approaching the 
horizon. To explain the reason of this, it must be observed 
that the French table was originally constructed for 32 0 of 
Fahrenheit, and was reduced to the mean temperature of 
50°, on the supposition that the refractions vary in the same 
proportion with the density of the air ; by which procedure the 
change in their quantity that arises from the variations of the 
