198 Mr. Groombridge’s Observations 
The table of mean refractions by the proposed formula, com- 
pared with the observed refraction, differs with the lowest 
star, less than 2"; but the refractions at those low altitudes 
having always been found uncertain, we may presume that 
the result so nearly approximates the truth, that we may not 
possibly ascertain the refractions more correctly. However, 
since my deduction will soon be submitted to the test of some 
excellent circular instruments, it will be seen, whether more 
accurate tables can be formed. 
M. Biot, in the report of the National Institute, observes, 
that a third correction of the refractions having been supposed 
to arise from the vapours at low altitudes, and therefore an 
equation might be found from the hygrometer,- he was in- 
duced to make some experiments with a prism containing 
warm air dried by potash, the outside of which was charged 
with the natural moisture of the atmosphere ; but he could 
discover no alteration in the refraction. He consequently in- 
fers, that the hygrometrical state of the atmosphere has no 
sensible effect on the refractions. Supposing an effect really 
to exist, a standard hygrometer would be required ; since 
those instruments, which show the comparative state of hu- 
midity or siccity of the atmosphere, give no scale of the velo- 
city of the current of air which causes the change. 
The mean astronomical refraction being determined in this 
manner, it will be further necessary to investigate the equa- 
tions for the barometer and thermometer; and I must pre- 
mise, that I have always applied those for the latter, from the 
temperature shewn by the thermometer attached to the tele- 
scope. Some astronomers have applied the correction for the 
external temperature, others for the mean of both ; but I believe 
