e sun. To this extent,
therefore, the sun-Jupiter system is analogous to the earth-moon system.
As the tides on the earth are driving away the moon, so the tides in
Jupiter and the sun are gradually driving the two bodies apart. But
there is a profound difference between the two cases. It can be proved
that the tides produced in Jupiter by the sun are more effective than
those produced in the sun by Jupiter. The contribution of the sun may,
therefore, be at present omitted; so that, practically, the
augmentations of the orbital moment of momentum of Jupiter are now
achieved at the expense of that stored up by Jupiter's rotation. But
what is 12 compared with 600,000. Even when the whole of Jupiter's
rotational moment of momentum and that of his satellites has become
absorbed into the orbital motion, there will hardly be an appreciable
difference in the latter. In ancient days we may indeed suppose that
Jupiter being hotter was larger than at present, and that he had
considerably more rotational moment of momentum. But it is hardly
credible that Jupiter can ever have had one hundred times the moment of
momentum that he has at present. Yet even if 1,200 units of rotational
momentum had been transferred to the orbital motion it would only
correspond with the most trivial difference in the distance of Jupiter
from the sun. We are hence assured that the tides have not appreciably
altered the dimensions of the orbit of Jupiter, or of the other great
planets.

The time will, however, come when the rotation of Jupiter on his axis
will be gradually abated by the influence of the tides. It will then be
found that the moment of momentum of the sun's rotation will be
gradually expended in increasing the orbits of the planets, but as this
reserve only holds about two per cent. of the whole amount in our system
it cannot produce any considerable effect.

The theory of tidal evolution, which in the hands of Professor Darwin
has taught us so much with regard to the past history of the systems of
satellites in the solar system, will doubtless also, as pointed out by
Dr. See, be found to account for the highly eccentric orbits of double
star systems. In the earth-moon system we have two bodies exceedingly
different in bulk, the mass of the earth being about eighty times as
great as that of the moon. But in the case of most double stars we have
to do with two bodies not very different as regards mass. It can be
demonstrated that the orbi