46 
POPULAR SCIENCE REVIEW. 
A fifth argument is derived from certain considerations 
depending on the behaviour of sun-raised cloud-masses in our 
own air, both with regard to the progress of the day, and with 
regard to the progress of the year. We know that speaking 
generally the clouds change as the day progresses, and that 
this is specially the case in those regions of the earth where 
regular zones exist. The sun, in tropical regions, rises in a 
clear sky and quickly gathers clouds together ; these remain 
till the afternoon, when they become dissipated (usually with 
violent disturbance, electrical and otherwise), and the stm sets 
in a clear sky. As seen from Venus or Mercury the cloud-belt 
would extend across the middle of the earth’s disc, but would 
not reach to the edge, either on the west or sun-rising side, or 
on the east or sun-setting side. Nothing of the kind is obser- 
vable in the cloud-belts of Jupiter. Not only do they extend 
right across (though becoming fainter near the edges because 
seen through deeper atmosphere), but cloud masses have been 
known to remain, quite recognizable in contour, during many 
Jovian days, and even for forty or fifty of our own much longer 
days. So also with regard to the year. In Jupiter’s case, 
indeed, the effect of annual changes in the arrangement of 
clouds would not be recognizable, simply because the planet’s 
equator is nearly coincident with the plane of Jupiter’s orbit. 
But in Saturn’s case the inclination of the equator is consider- 
able ; so that, as seen from the sun, the equator passes far to 
the north and far to the south of the centre of the disc, 
during the summer of the northern and southern hemispheres, 
respectively. We should expect to find these changes accom- 
1 panied by corresponding changes in the position of the central 
zone of clouds. Our terrestrial tropical cloud-zone, being sun- 
raised, follows the sun, passing north of the equator during our 
northern summer, until at midsummer it reaches the tropic of 
Cancer, and passing south of the equator during the southern 
summer, until at midsummer (December) it reaches the tropic 
of Capricorn. But instead of the mid-zone of Saturn behaving 
in this way, it remains always equatorial. 
Another (the sixth) argument, and in my opinion an argu- 
ment altogether irresistible, is derived from the changes which 
have taken place from time to time in the outline of the 
planets Jupiter and Saturn, unless observations made by most 
skilful astronomers, and with instruments of considerable power, 
are to be rejected as unworthy of trust. I refer in particular, 
first to the observations by Admiral Smyth, Sir K. Maclear, 
and Professor Peacock, of the reappearance of the second 
satellite of Jupiter a few minutes after it had apparently made 
its complete entry upon the planet’s disc at the beginning of a 
transit ; and secondly, to the fact that Sir W. and Sir J. Her- 
