1875.] The Atmospheres of the Planets. 467 
comparative slight influence of very different laws of decrease 
of temperature. Adopting 28’ 53” for the horizontal refrac- 
tion corresponding to a density equal to that of the earth’s, 
the true surface density of the atmosphere of Venus obtained 
from the value 44’ 30” for the horizontal refraction is=1°546. 
This corresponds to a pressure (measured in the height of 
the barometer on the earth) of 1175 millimetres of mercury, 
or 46°4 inches. 
The surface density of the atmosphere of Venus being 
thus determined, its variation with the height above the 
surface can be ascertained. At 3°67 miles above the surface 
the density would be only half that at the surface, at seven 
miles rather less than one-fourth, and at 11 miles only one- 
twelfth ; which, corresponding to a height of nearly eight 
milesonthe earth, would be probably the limit ofthe great dense 
cloud-bearing strata of Venus’s atmosphere. At a height of 
20 miles, the density of the atmosphere would be only one- 
hundredth that at the surface, aheight of 30 miles would 
reduce it to one-thousandth, 39 miles to one-ten-thousandth, 
and 58 miles to one-millionth, where it may be regarded as 
sensibly becoming evanescent. Under the most favourable 
condition,—it requiring over 100 miles to subtend one-second 
of arc at the distance of Venus,—the insignificant nature of 
the breadth of the layer of the atmosphere of Venus is 
apparent, and the great difficulty of detecting it manifest. 
Ninety-nine-hundredths of the atmosphere of Venus lies 
within one-fifth of a second of arc from the limb of the 
planet, and is under ordinary conditions absolutely indistin- 
guishable from it; and almost the entire remaining portion 
lies within half a second of arc, the whole atmosphere be- 
coming sensibly evanescent at a distance of two-thirds of a 
second. 
Inoccultations of starsby Venus—a phenomenon comipara- 
tively rarely visible—as no sensible refraction would ensue 
from the effects of Venus’s atmosphere until the star was 
within a third of a second of arc from the planet, it would 
be utterly undetectable at that distance, for the brightness of 
the planet is so great as to cause an amount of irradiation at 
the limb far greater than this, so that the star would disap- 
pear at the bright fictious limb, as if no atmosphere existed ; 
but at the dark limb of the planet under favourable condi- 
tions a beautiful and delicate ring of light might be 
momentarily detected flashing round the dark body of the 
planet before immersion. 
Considerable discussion has arisen in connection with the 
observations of the late transit of Venus from the observation 
