366 Astronomical and Nautical Collections. 



required for multiplying the numbers of the table ; and in this man- 

 ner he has obtained for Bradley's Table, first reduced on account 

 of the sun's parallax, the factor 1.02845 and has proposed still fur- 

 ther to improve it by adopting the form 



7- = 58.1192 ta (Z. .D. - 3.3625;). 

 Mr. Groombridge has not recorded the particular temperatures 

 of his observations, but has reduced them to the mean temperature 

 of the table, which is supposed to be 49° for the interior thermo- 

 meter, and 45° for the exterior. The results may, however, be of 

 use in continuing upwards the Empirical Table, inserted in a 

 former number of these Collections (XIII) from Mr. Groombridge's 

 later observations, and it will be perfectly justifiable to divide the 

 sum of the two refractions in the ratio of the corresponding re- 

 fractions of any approximate table, in order to determine the larger 

 of the two with little chance of error. In this manner we may 

 obtain the following Table, selecting the observations, at conve- 

 nient altitudes, which have been most frequently repeated. 



Stars. Obs. Alt. Refr. N. A. . Diff. 



Mr. Groombridge has also taken some pains to ascertain from 

 observation, the magnitude of the thermometrical correction, though 

 without distinguishing the different effects at different altitudes ; 

 and he finds for the exterior thermometer .0021 for every degree 

 of variation reckoned from 45°, and .0023 or .0024 for the degrees 

 of the interior thermometer reckoned from 49°. His own formula 

 is thus compared with the French Tables, and with Piazzi's empi- 

 rical correction. Barometer 29.6. 



