204 ASTEONOMY. 



distance of Mars from tbe sun. This result is worthy of notice in con- 

 nection with the fact that the inner satellite of Mars revolves with a 

 periodic time much shorter than that of the planets' rotation ; for (as 

 suggested in a previous paper) solar tidal friction will have been com- 

 petent to reduce the planetary rotation without directly affecting the 

 satellites' orbital motion. 



It is, then, shown to be probable that solar tidal friction was a more 

 important cause of change when the planets were less condensed than 

 it is at present. Thus we are not to accept the present rate of action of 

 solar tidal friction as indicating that which has been held true in all 

 past time. 



It is ■also shown that if a planetary mass generates a large satellite, 

 the planetary rotation is reduced after the change more rapidly than 

 before ; nevertheless the genesis of such a satellite is preservative of tlie 

 moment of momentum, which is internal to the planetary subsystem. 

 This conclusion is illustrated by the comparatively slow rotation of the 

 earth, and by the large amount of angular momentum residing in the 

 system of moon and earth. 



An examination of the manner in which the difference of distances, 

 of the various planets from the sun will have affected the action of tidal 

 friction, leads to a cause for the observed distribution of satellites in the 

 solar system. 



According to the nebular hypothesis a planetary mass contracts, an<J 

 rotates quicker as it contracts. The rapidity of its revolution causes 

 its form to become unstable, or perhaps, as seems more probable, an 

 equatorial belt gradually detaches itself; it is immaterial which of these 

 really takes place. In either case the separation of that part of the 

 mass which before the change had the greatest angular momentum per- 

 mits the central portion to resume a planetary shape. The contraction 

 and increase of rotation proceed continually until another portion is de- 

 tached, and so on. There thus recur at intervals a series of epochs of 

 instability or abnormal change. Now, tidal friction must diminish the 

 rate of increase of rotation due to contraction, and therefore if tidal fric- 

 tion and contraction are at work together the epochs of instability must 

 recur more rarely than if contraction acted alone. If the tidal retarda- 

 tion is sufiicieutly great, the increase of rotation dne to contraction will 

 be so far counteracted as never to permit an epoch of instability to occur. 

 The rate of solar tidal friction decreases rapidly as we recede from the 

 r^un, and therefore these considerations accord with w^hat we observe 

 in the solar system. For Mercury and Venus have no satellites, and 

 there is a progressive increase in the number of satellites as we recede 

 from the sun. 



Whether this be the true cause of the observed distribution of satel- 

 lites amongst the planets or not, it is remarkable that the same cause 

 also affords an exi)lanation of that difference between the tellurian sys- 

 tem and the other planets with their satellites which has permitted tidal 



