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a velocity of some two hundred and sixty miles per hour greater
than the red spot。 Denning obtained one hundred and sixty…nine
observations of this bright marking during the years 1880…1883；
and determined the period as nine hours； fifty minutes； eight and
seven tenths seconds （five and a half minutes less than that of
the red spot）。 Although the latter is now somewhat faint； the
white spot gives promise of remaining visible for many years。
During the year 1886 a large number of observations of Jupiter
were made at the Dearborn Observatory； Chicago； U。 S。； by Prof。
G。 W。 Hough； using the eighteen…and…a…half…inch refractor of the
observatory。 Inasmuch as these observations are not only of high
intrinsic interest； but are in conflict； to some extent； with
previous records； a somewhat full abstract of them will be
useful： The object of general interest was the great red spot。
The outline； shape； and size of this remarkable object has
remained without material change from the year 1879； when it was
first observed here； until the present time。 According to our
observations； during the whole of this period it has shown a
sharp and well…defined outline； and at no time has it coalesced
or been joined to any belt in its proximity； as has been alleged
by some observers。 During the year 1885 the middle of the spot
was very much paler in colour than the margins； causing it to
appear as an elliptical ring。 The ring form has continued up to
the present time。 While the outline of the spot has remained
very constant； the colour has changed materially from year to
year。 During the past three years （1884… '86） it has at times
been very faint； so as barely to be visible。 The persistence of
this object for so many years leads me to infer that the formerly
accepted theory； that the phenomena seen on the surface of the
planet are atmospheric； is no longer tenable。 The statement so
often made in text…books； that in the course of a few days or
months the whole aspect of the planet may be changed； is
obviously erroneous。 The oval white spots on the southern
hemisphere of the planet； nine degrees south of the equator； have
been systematically observed at every opposition during the past
eight years。 They are generally found in groups of three or
more； but are rather difficult to observe。 The rotation period
deduced from them is nearly the same as from the great red spot。
These spots usually have a slow drift in longitude of about five
seconds daily in the direction of the planet's rotation； when
referred to the great red spot； corresponding to a rotation
period of twenty seconds less than the latter。'
〃This shows；〃 continued Bearwarden； 〃that as long ago as towards
the close of the nineteenth century the old idea that we saw
nothing but the clouds in Jupiter's atmosphere was beginning to
change； and also how closely the two English writers and Prof。
Hough were studying the subject； though their views did not
entirely agree。 A white spot is merely a storm…centre passing
round and round the planet； the wind running a little ahead of
the surface； which accounts for its rapid rotation compared with
the red spot； which is a fixture。 A critic may say we have no
such winds on earth； to which I reply； that winds on a planet of
Jupiter's size； with its rate of rotationthough it is
480；000；000 miles from the sun and the internal heat is so near
the surfaceand with land and water arranged as they are； may
and indeed must be very different from those prevailing on earth；
the conditions producing and affecting them being so changed。
Though the storm…centre moves two hundred and sixty miles an
hour； the wind need not blow at that rate。〃
Later they saw several smaller spots drifting eastward； but
concluded that any seaworthy ship might pass safely through them；
for； though they were hurricanes of great violence； the waves
were small。
〃There would be less danger；〃 said Bearwarden； 〃of shipping seas
here than there is on earth； the principal risk to travellers
would be that of being blown from the deck。 On account of the
air's weight in connection with its velocity； this would
necessitate some precaution。〃
The next object of interest was the great red spot。 It proved；
as Cortlandt had predicted； to be a continent； with at that time
no special colour； though they easily recognized it by comparing
its outlines with those of the spot in the map。 Its length； as
they already knew； was twenty…seven thousand miles； and its
breadth about eight thousand miles； so that it contained more
square miles than the entire surface of the earth； land and water
included。
〃It is clear；〃 said Cortlandt； 〃that at some season of Jupiter's
long year a change takes place that affects the colour of the
leavessome drought or prolonged norther； for it is obvious that
that is the simplest explanation。 In like manner we may expect
that at some times more white spots will move across the ocean
than at others。〃
〃On account of the size of these continents and oceans；〃 said
Bearwarden； 〃it is easy to believe that many climatic conditions
may prevail here that can scarcely exist on earth。 But what a
magnificent world to develop； with its great rivers； lakes； and
mountains showing at even this distance； and what natural
resources must be lying there dormant； awaiting our call！ This
constantly recurs to my mind。 The subjugation and thorough
opening up of this red spot continent will probably supply more
interesting problems than straightening the axis of the earth。〃
〃At our next visit；〃 replied Ayrault； 〃when we have established
regular interplanetary lines of travel； we may have an
opportunity to examine it more closely。〃 Then they again
attracted the nearest moon beyond which they had swung； increased
the repulsion on Jupiter； and soared away towards Saturn。
〃We have a striking illustration of Jupiter's enormous mass；〃
said Cortlandt； as the apparent diameter of the mighty planet
rapidly decreased； 〃in the fact that notwithstanding its numerous
moons； it still rotates so rapidly。 We know that the earth's
days were formerly but half or a quarter as long as now； having
lasted but six or eight hours。 The explanation of the elongation
is simple： the earth rotates in about twenty…four hours； while
the moon encircles it but once in nearly twenty… eight days； so
that our satellite is continually drawing the oceans backward
against its motion。 These tidal brakes acting through the
friction of the water on the bottom； its unequal pressure； and
the impact of the waves on the shore； are continually retarding
its rotation； so that the day is a fraction of a second longer
now than it was in the time of Caesar。 This same action is of
course taking place in Jupiter and the great planets； in this
case there being five moons at work。 Our moon； we know； rotates
on its axis but once while it revolves about the earth； this
being no doubt due to its own comparative smallness and the great
attraction of the earth； which must have produced tremendous
tides before the lunar oceans disappeared from its surface。〃
In crossing the orbits of the satellites； they passed near
Ganymede； Jupiter's largest moon。
〃This；〃 said Cortlandt； 〃was discovered by Galileo in
1610。 It is three thousand four hundred and eighty miles
in diameter； while our moon is but two thousand one
hundred and sixty； revolves at a distance of six hundred
and seventy…eight thousand three hundred miles from
Jupiter； completes its revolution in seven days and four
hours； and has a specific gravity of 1。87。〃
In passing； they observed that Ganymede possessed an atmosphere；
and continents and oceans of large area。
〃Here；〃 said Bearwarden； 〃we have a body with a diameter about
five hundred miles greater than the planet Mercury。 Its size；
light specific gravity； atmosphere； and oceans seem to indicate
that it is less advanced than that planet； yet you think Jupiter
has had a longer separate existence than the planets nearer the
sun？〃
〃Undoubtedly；〃 said Cortlandt。 〃Jupiter was condensed while in
the solar…system nebula； and began its individual existence and
its evolutionary career long before Mercury was formed。 The
matter now in Ganymede； however； doubtless remained part of the
Jupiter…system nebula till after Mercury's creation； and； being
part of so great a mass； did not cool very rapidly。 I should say
that this satellite has about the same relation to Jupiter that
Jupiter has to the sun； and is therefore younger in point of time
as well as of development than the most distant Callisto； and
older； at all events in years； than Europa and Io； both of which
are nearer。 This supposition is corroborated by the fact that
Europa； the smallest of these four； is also the densest； having a
specific gravity of 2。14； its smallness having enabled it to
overtake Ganymede in development； notwithstanding the latter's
start。 In the face of the evidence before us we must believe
this； or else that； perhaps； as in the case of the asteroid
Hilda； something like a collision has rejuvenated it。 This might
account for its size； and for the Nautical A