466 The Atmospheres of the Planets. (October, 
essentially analogous to our owa. The amount of the hori- 
zontal refraction, fully one-sixth greater than our own, indi- 
cates that the density of the atmosphere of Venus must 
exceed ours; whilst the dense mass of clouds, by retarding 
radiation, must render the decrease of temperature compara- 
tively considerably slower, and therefore in computing the 
amount of the horizontal refraction due to any atmosphere 
this must be taken into account. It has been shown by the 
investigations of Laplace, Ivory, Bessel, and Lubbock, that 
the influence of the law of decrease of temperature on the 
amount of refraétion due to an atmosphere of given surface 
density is of comparatively secondary importance, so that 
the uncertainty that must be considered to exist with regard 
to this point will not prevent a tolerably accurate value for 
the surface density of the atmosphere being obtained. The 
great similarity between the physical conditions prevailing 
on the surface of Venus to those on the earth, in as far as 
they effect the atmospherical laws, renders it probable that 
the law of decrease of temperature observed will not be 
materially different from that on the earth, though, perhaps, 
slightly slower, whilst the greater density and the number of 
clouds may, with the greater proximity of the planet to the 
sun, render the mean temperature of Venus sensibly greater. 
than the earth’s. Returning, therefore, to the series of equa- 
tions (12) to (15) itis necessary to ascertain the horizontal 
refraction due to the passage of a ray of light through such 
an atmosphere. The long and complicated nature of this 
prevents this being effected here, owing to the very consider- 
able amount of horizontal refraction rendering it necessary 
to take into account a whole series of terms not necessary: 
even on the earth, and the resulting expression for the hori- 
zontal refraction contains no less than twenty-three terms. 
Supposing, however, the value of the constant f to be one- 
third, or slightly greater than on the earth, owing to the prob- 
ably higher temperature of Venus, whilst putting ,=0'9, so 
as to ensure a slightly slower decrease of temperature, then 
for a surface density the same as, and an atmosphere of simi- 
lar nature to the earth, the horizontal refraction on Venus, 
would be— 
% = 1733° = 28’ 53” 
By assuming Ivory’s theory to hold, the result would be 
28’ 55”, or sensibly identical, though the rate of decrease 
of temperature is sensibly different. Bessel’s hypothesis, 
gives 299”, and the assumption of a uniform temperature 
which gives the maximum refraction 29’ 25”; showing the 
