426 Mr. Ivory on the astronomical refractions. 
the number of De Lambre, which is that employed in the 
calculation of the French tables of refraction, we get 
a = — -000293876 ; 
I 4- K .p 
and, by reducing to the mean temperature of io° on the cen- 
tigrade scale, or 50° of Fahrenheit, and to the standard 
barometer 30 English inches, we finally obtain 
a — .0002835, 
Log. — 4 . 4525531 . 
From a numerous set of observations Dr. Brinkley has de- 
duced a value somewhat less than the preceding ; and hence 
it appears, that there is still some small degree of uncertainty 
in the determination of this coefficient. It is to be expected 
that the unequal mixture of moisture, by altering the density 
of the air, will produce variations in the value of a. But it 
has been determined that, when a quantity of aqueous vapour 
is added to a volume of air, the density is diminished nearly 
in the same proportion that the refractive power of the 
vapour is greater than the refractive power of the air. A com- 
pensation is thus effected ; and the mixed medium is hardly 
different from dry air of the like density in its action on light. 
- The value of / must be found by means of the formula 
p = l x ( 1 + /3t ) £. 
Here we must conceive that ^ is measured in parts of the 
density of mercury ; and, as ( 1 -j- jS-r ) ^ is the density of the 
air reduced to the fixed temperature zero, the equation merely 
expresses that the density of air is proportional to the pres- 
sure when the temperature remains unchanged. Now, M. M. 
Biot and Arago have found that the specific gravity of air 
under the pressure of 0.76 metres, and at the temperature of 
