168 On atmospherical Refraction. © 
may have adjusted them to represent some good Table of Re- 
fractions, as that of the French astronomers: he may have em- 
ployed for the same purpose a great number of accurate observa- 
tions; he may have had recourse to both these methods. 
The Table in the Nautical Almanac is easily compared with 
that in the Connaissance des Tems. Both suppose the same mean 
temperature, 50° of Fahrenheit, and 10°, of the centigrade scale. 
In the English Table the mean pressure of the atmosphere is 
taken at 30 inches; in the French Table, at 0°76 metres, or 
29-92 inches. The numbers in the two tables will therefore be 
brought to the same circumstances, if those in the French Table 
: 8 1 . te 
be increased by the —>> or >> part. When this is done the 
tables will stand as below: _ Sl ae 
Altitudes. Conn. des Tems. Naut. Alm, 
OF -..) oe SOMMG see BBS G1% 
BT AB) Bay sa ae 
Bes PO aR Seog 
Eh ep Ls Dat ae 
2.0 8. 18 25° see 
ar... 016" 16) it went ay 
Diego ae SR ue 
E>: Se i 3s 19) ae eee aa te a 
Ab eglics ANON Aber ne 
Eee ACG AD ok ye oe 
B aiddes) ‘Bitter QahGA "HORR: cetera 
In the remaining parts the two tables agree perfectly with one 
another. It appears therefore that the French Table is very ac- 
eurately represented by Dr. Young’s formula, the greatest dif- 
ference being no more than 4” or 5” at low altitudes betweer 
1 and 4 degrees. And in like manner, there can be no doubt, 
a similar formula may be so adjusted as to represent with equa! 
exactness the Table of Bradley, or any other Table of Refrac- 
tions. 
It would be extremely important to be informed, whether a 
great number of good and original astronomical observations has 
been employed in constructing the English Table, and what those 
observations are. If this has actually been the case; if the Eng- 
lish Table has a real and solid foundation different from the 
Table in the Connaissance des Tems ; it must be allowed that no 
greater or more honourable testimony can be given in favour of 
the accuracy of the labours of the French astronomers. 
Sept. 4, J821. J. Ivory. 
XXXVIII. On 
