1875.] The Atmospheres of the Planets. 457 
of a uniform temperature. Therefore the maximum 
depth any planetary atmosphere can possess will be that 
when its temperature is uniform. Now, applying this to 
the three great planets, Jupiter, Saturn, and Uranus, and 
allowing that their surface may be so intensely heated as to 
be at a temperature of r100° C., and the computation of 
their depths on the hypothesis of a uniform temperature 
will give their maximum depths. 
On Jupiter, under these conditions, commencing from a 
point already mentioned, when considering their depth at a 
temperature near zero centigrade, then at 74 miles below 
the upper side of the visible cloud layers it would be ten 
times denser than our own atmosphere, at 110 miles com- 
pressed until denser than water, and at only 148 miles as 
dense as mercury; though, as before remarked, long before 
this could have happened, the condensation of its consti- 
tuents must have occurred, as the whole ceased to be a gas. 
On Saturn, the depths beneath the surface, where the same 
would occur, would be 154, 231, and 308 miles, and on 
Uranus 178, 268, and 357 miles. The telescopic insignifi- 
cance of the depths, in mere dimensions, of the greatest of 
these three quantities on each planet is best shown by con- 
sidering that on Jupiter it subtends an angle of only 0°06” of 
arc, on Saturn of 0°08,” and on Uranus of 0°04’—quantities 
unmeasurable, and almost invisible even in the finest tele- 
scopes. It cannot be supposed, however, that these dimen- 
sions, small as they are, can ever be reached by any of the 
three planets, for a temperature such as supposed could not 
extend undiminished to the outer limits of the cloud-bearing 
strata; and, therefore, to obtain in any way the probable 
depth of the atmospheres to these planets use must be 
made of equations (15) and (13). To take the most favour- 
able circumstance possible, suppose the temperature never 
to fall below zero C., then the value of f from (16) for the 
supposed intense surface heat of the planets will be o°8o, 
whilst to give a very favourable condition to a great depth, 
make ¢ instead of near unity only 0°20; so that temperature 
decreases so slowly as to be even beyond the summit of the 
cloud-bearing strata, the atmosphere will be of still consider- 
able warmth. Then the three respective points already 
mentioned would, on Jupiter, be reached at a depth of 60, 
972, and 78 miles; on Saturn, at 126, 152, and 164 miles; 
and on Uranus at 144, 173, and 188 miles respectively, or 
scarcely one-half of that found on the previous assumption. 
These depths appear, therefore, to be the maximum possible 
on any permissible condition on these three giant planets of 
the solar system. 
