CONDITION OF THE LARGER PLANETS. 
47 
schel, Sir Gr. Airy, the Bonds and Coolidge in America, and 
several of the Greenwich observers, have recognized the occa- 
sional assumption by Saturn of what is commonly called his 
u square shouldered ” aspect. These observations are far too 
well-authenticated, and were made by observers far too skilful, 
to be open to doubt or cavil. They cannot possibly be explained 
except by assuming that the outlines of Jupiter and Saturn are 
variable to such an extent that the variations appreciably affect 
the figure of the planets. Such variations, involving differences 
of level of two or three thousand miles, are utterly incredible, 
and in point of fact impossible, in the case of planets like our 
earth. The heat generated by such changes would of itself 
suffice to melt and in large degree to vaporize the crust for 
many thousands of square miles around the scene of upheaval 
or depression, so that we should thus have, but in another way, 
the heat which my theory indicates. On the other hand, such 
changes of outline in a planet whose apparent outline is not 
formed by its real surface, but by cloud layers thousands of 
miles above the real surface, are very easily explained. Nay, 
they are to be expected (though only as rare phenomena). 
We know that cloud-belts sometimes form, or are dissipated, 
rapidly on the face of the disc. Equally, therefore, they must 
sometimes form or become dissipated rapidly at parts of the 
planet so placed as to form the apparent outline. There would 
then be a rapid change of outline, such as must have occurred in 
the case of the apparent reappearance of Jupiter’s second satellite. 
Slower changes in the cloud-belts would correspond to the 
changes of shape observed in Saturn’s case, and would explain 
Schroter’s observation that at times the outline of Jupiter has 
seemed to him irregular, as if the planet’s surface were partially 
flattened. Other observations tending in the same direction, as 
peculiarities in the shape of the shadows of Jupiter’s satellites 
on the planet, in the shape of Saturn’s shadows on his rings, 
and so on, are of less weight perhaps than those already con- 
sidered, but unless those who recorded them (including some of 
the most skilful observers known) were entirely deceived, such 
observations can only be fully explained by the great depth of the 
cloud-laden atmosphere which surrounds the giant planets. 
Lastly, there is the argument derivable from the peculiar 
brightness of the planets Jupiter and Saturn. These planets 
might be so hot as to glow with an intense light and heat, yet 
no part of their light might be discernible, the deep cloud- 
layers simply cutting it off before it reached the outermost or 
visible cloud surface. Or this might happen with all the rays 
except those which travelled the shortest way through the 
cloud-layers. In the former case we should perceive some of 
the inherent light of these planets, in the latter we should only 
