516 
MR, G. H. DARWIN OR THE 
sun in studying the possible changes in the planetary orbits as resulting from tidal 
friction. 
But the difference of physical condition in the several planets is probably consider¬ 
able, and this would lead to differences in the coefficients of tidal friction to which 
there is no apparent means of approximating. It therefore seems inexpedient at 
present to devote time to the numerical solution of the problem of the rigid sun and 
the tidally-disturbed planets. 
§ 7. Numerical data and deductions therefrom. 
Although we are thus brought to admit that it is difficult to construct any problem 
which shall adequately represent the actual case, yet a discussion of certain numerical 
values involved in the solar system and in the planetary subsystems will, I think, lead 
to some interesting results. 
The fundamental fact with regard to the theory of tidal friction is the transforma¬ 
tion of the rotational momentum of the planet as it is destroyed by tidal friction into 
orbital momentum of the tide-raising body. 
Hence we may derive information concerning the effects of tidal friction by the 
evaluation of the various momenta of the several parts of the solar system. 
Professor J. C. Adams has kindly given me a table of values of the planetary 
masses, each with its attendant satellites. The authorities were as follows: for 
Mercury, Encke; for Venus, Le Verrier ; for the Earth, Hansen ; for Mars, Hall; 
for Jupiter, Bessel; for Saturn, Bessel; for Uranus, Von Asten ; for Neptune, 
Newcomb. 
The masses were expressed as fractions of the sun. The results, when earth plus 
moon is taken as unity, are given in the table below. The mean distances, taken from 
Herschel’s ‘ Astronomy,’ are given in a second column. 
Masses (to.). 
Mean Distances (c.). 
Sun . 
315,51U 
Mercury. 
•06484 
•387098 
Venus 
•78829 
•723332 
Earth 
1-00000 
1-000000 
Mars . 
•10199 
1-523692 
Jupiter . 
30P0971 
5-202776 
Saturn . 
90-1048 
9 '538786 
Uranus . 
14-3414 
19-18239 
N eptune. 
16-0158 
30-05660 
The unit of mass is earth plus moon, the unit of length is the earth’s mean distance 
from the sun, and the unit of time will be taken as the mean solar day. 
