454 The Atmospheres of the Planets. [October, 
limited atmospheres of these three giant planets can possess; 
and their insignificance is such that on the limb they would 
subtend an angle such a minute fraction of a second of arc 
as to be almost undetectable and quite unmeasurable. Yet as 
faras the observed phenomena of the cloud belts of Jupiter are 
concerned, the depth is ample to explain all that has yet 
been definitely ascertained. Considering these three planets 
from the point of view that has hitherto been adopted, these 
may be regarded as the maximum depths of their cloud- 
bearing atmospheric envelopes, because any law of decrease 
of temperature that may be adopted will be without sen- 
sible effect on these results. But there is another point 
from which Jupiter at least may be regarded, and which 
considers that perhaps this giant planet may have retained 
sufficient heat to entirely remove the conditions prevailing 
on its surface from any analogy in point of temperature to 
those onthe earth. It has been urged with much force that, 
from its great mass, Jupiter may have been much longer in 
cooling down than the earth and other smaller planets, and 
that, therefore, its surface temperature may still be very 
high; perhaps approaching or even reaching a low red heat. 
Under these conditions the law of decrease of temperature 
rises so much into importance that it cannot remain neg- 
lected, as has hitherto been done; but some attempt must 
be made to take it into account. The researches into the 
spectrum of Jupiter show that the greater majority, if not 
all, the light it sends to the earth, is merely reflected sun- 
light that has traversed a sufficient layer of atmosphere to 
exhibit absorption lines which indicate the presence of 
aqueous vapour. Had Jupiter been actually incandescent, 
that is to say were its surface, which can be probably seen 
through the clouds environing it, so intensely heated as to 
become white hot, a very different result would have been 
expected, and only one uniform mass of permanent dense 
cloud would be shown by Jupiter. Moreover, the formation 
of persistent marking in the visible surface of the planet, the 
arrangement noticeable in their form and position, and their 
appearance all appear more consistent with a moderate 
surface temperature, and could hardly be expected to be 
screening a white-hot incandescent planet. Some striking 
evidence of the vast energies at work could not but become 
manifest in a most marked manner were the entire surface 
of Jupiter one molten and seething mass of fire, as such a 
supposition would render it, whilst the environing vapours 
would become the scene of titanic throes from the alternate 
decomposition and re-combination of those energetic elements 
