We thus see that the tides do not appear to have exercised anything like the same influence in the affairs of our solar system generally which they have done in that very small part of the solar system which consists of the earth and moon. This is, as I have endeavoured to show in these lectures, the scene of supremely interesting tidal phenomena; but how small it is in comparison with the whole magnitude of our system may be inferred from the following illustration. I represent the whole moment of momentum of our system by L1,000,000,000, the bulk of which is composed of the revolutional moments of momentum of the great planets, and the rotational moment of momentum of the sun. On this scale the rotational share which has fallen to our earth and moon does not even rise to the dignity of a single pound, it can only be represented by the very modest figure of 19_s._ 5_d._ This is divided into two parts--the earth by its rotation accounts for 3_s._ 4_d._, leaving 16_s._ 1_d._ as the equivalent of the revolution of the moon. The other inferior planets have still less to show than the earth. Venus can barely have more than 2_s._ 6_d._; even Mars' two satellites cannot bring his figure up beyond the slender value of 1-1/2_d._; while Mercury will be amply represented by the smallest coin known at her Majesty's mint. The same illustration will bring out the contrast between the Jovian system and our earth system. The rotational share of the former would be totally represented by a sum of nearly L12,000; of this, however, Jupiter's satellites only contribute about L89, notwithstanding that there are four of them. Thus Jupiter's satellites have not one hundredth part of the moment of momentum which the rotation of Jupiter exhibits. How wide is the contrast between this state of things and the earth-moon system, for the earth does not contain in its rotation one-fifth of the moment of momentum that the moon has in its revolution; in fact, the moon has gradually robbed the earth, which originally possessed 19_s._ 5_d._, of which the moon has carried off all but 3_s._ 4_d._ And this process is still going on, so that ultimately the earth will be left very poor, though not absolutely penniless, at least if the retention of a halfpenny can be regarded as justifying that assertion. Saturn, revolving as it does with great rapidity, and having a very large mass, possesses about L2700, while Uranus and Neptune taken together would figure for about the same amount.