drawn into the sun; while Neptune, seventy-five times as far off,
and hence attracted only 1/5626 as much, must be slowed down to 3.4
miles a second to prevent its flying away from the feebler
attraction of the sun. The orbital velocity of the various planets
in miles per second is as follows:

Mercury 29.55 | Jupiter 8.06
Venus 21.61 | Saturn 5.95
Earth 18.38 | Uranus 4.20
Mars 14.99 | Neptune 3.36

Hence, while the earth makes one revolution in its year, Mercury
has made over four revolutions, or passed through four years; the
slower Neptune has made only 1/164 of one revolution.

The time of axial revolution which determines the length of the
day varies with different planets. The periods of the four planets
nearest the sun vary only half an hour from that of the earth,
while the enormous bodies of Jupiter and Saturn revolve in ten
and ten and a quarter hours respectively. This high rate of speed,
and its resultant, centrifugal force, has aided in preventing these
bodies from becoming as dense as they would otherwise be--Jupiter
being only 0.24 as dense as the earth, and Saturn only 0.13. This
extremely rapid revolution produces a great flattening at the poles.
If Jupiter should rotate four times more rapidly than it does, it
could not be held together compactly. As it is, the polar diameter
is five thousand miles less than the equatorial: the difference
in diameters produced by the [Page 102] same cause on the earth,
owing to the slower motion and smaller mass, being only twenty-six
miles. The effect of this will be more specifically treated