Nebular Theory. — Mistockles. 3O7 
farther away from the Sun, until they have now been found 
to exist near to the orbit of Jupiter. There is, therefore, no 
reasonable ground on which any one can claim, that such plan- 
ets may not exist beyond thie most distant one which has been 
discovered. This forces us to admit, that minor planets may 
be found outside as well as inside the orbit of Jupiter. When 
it is admitted, further, that if a small planet would intersect 
or reach the orbit of Jupiter, the result would be, that the less- 
er planet would be attracted to the larger at a point where 
thieir orbits cross each other or somewhere in that vicinity, 
then it is practically admitted, that this is the manner in which 
this mightly planet may have received its many moons. 
It may be of interest now to find out how near one of the 
minor planets might approach one of the great ones, Saturn, 
for instance, in order to be captured by it. Astronomers have 
figured out mathematically that one of Saturn's moons would 
have to recede a distance of 30 million miles before the planet 
would lose it. It follows, then, that a small planet coming 
within that distance would be attracted by Saturn. 
It may be, that this distance is rather large, and we shall 
therefore, try to determine it also on a different basis and 
compare the result with that given above. Let us consider, 
for this purpose, the relation in which the velocity and dis- 
tance of Neptune stand to the distance and motion of IMer- 
cury, which will give us the diminution of the Sun's attraction 
in proportion to the increase of the distance. Next we mav 
apply the same principle to Saturn's moons, and thus deter- 
mine fairly accurately the limits which we wish to know. The 
distance of Neptune is yy times that of Mercury, the velocitv 
of the latter is 29 English miles a second and that of the for- 
mer 3.2. This shows that the power of the Sun's attraction in 
the orbit of Neptune has fallen to one-ninth of what it is in 
Mercury's orbit. In considering the satellites of Saturn, let 
us take, for instance, the fifth one, Rhea. This has a distance 
of 336,000 miles, according to Flamarion, and a sidereal per- 
iod of 4 days, 12 hours and 25 minutes, which gives it a veloc- 
ity of 323.5 EngHsh miles a minute. By multiplying the given 
distance by yy we find the distance from the planet to be 25,- 
872,000 miles. At this distance the power of attraction has, 
thus, fallen to one-ninth of what is in Rhea's orbit. Further, 
