390 Scientific Proceedings, Royal Dublin Society. 
Apart from any theory as to the nature and mode of genesis 
of these spots, whether they be due to upheavals of the matter 
forming the visible surface of Jupiter in a manner somewhat 
analogous to the origin of sunspots, or to the formation of dark 
clouds in his atmosphere, or to the temporary withdrawal of a 
heavy pall of vapours, thus forming a rift through which we get a 
glimpse of the interior parts of the planet, it will be obvious that 
observations of the movements of such spots must be of great 
value, as enabling us to determine the elements of the rotation ~ 
of the planet—the position of its polar axis, and the time in 
which it completes a rotation. 
Since the time of Schroeter the movements of many such spots 
have been observed, and the curious result is arrived at, that 
different parts of the surface rotate at different rates. 
Tt will of course be understood that this anomalous behaviour 
is quite independent of the fact, that the equatorial parts move 
with a higher linear velocity than those nearer the poles, a law 
which must hold for any rotating solid sphere such as the Harth or 
Mars. But in the case of Jupiter it is found that the different 
spots complete a rotation in different periods, a state of affairs 
which is quite incompatible with their being each rigidly attached 
to a solid rotating body. 
In this respect Jupiter resembles the Sun itself, where the rate 
of movement of spots on the whole diminishes as the spot is more 
and more removed from the equator. 
In the case of Jupiter this variation is scarcely so regular as 
in that of the Sun, but there are distinct differences in the periods 
of rotation of different zones which conspire with other facts 
in regard to its physical character, such as its low density 
(less than one quarter that of the Harth and only 1:29 compared 
with that of water), and the marked polar compression of its 
globe, in leading us to the conclusion, that we are here 
dealing with a body, to a great extent at least, in a state of 
vapour. 
As was pointed out recently by Mr. A. Stanley Williams in the 
Monthly Notices of the Royal Astronomical Society, there are nine 
distinctly marked zones, whose periods of rotation have been well 
determined. 
