upiter at least, there is no uncertainty about the result.

The task is peculiarly simplified in the case of the greatest planet of
our system by the beautiful system of moons with which he is attended.
These little moons revolve under the guidance of Jupiter, and their
movements are not otherwise interfered with so as to prevent their use
for our present purpose. It is from the observations of the satellites
of Jupiter that we are enabled to measure his attractive power, and
thence to calculate the mass of the mighty planet.

To those not specially conversant with the principles of mechanics, it
may seem difficult to realise the degree of accuracy of which such a
method is capable. Yet there can be no doubt that his moons inform us of
the mass of Jupiter, and do not leave a margin of inaccuracy so great as
one hundredth part of the total amount. If other confirmation be needed,
then it is forthcoming in abundance. A minor planet occasionally draws
near the orbit of Jupiter and experiences his attraction; the planet is
forced to swerve from its path, and the amount of the deviation can be
measured. From that measurement the mass of Jupiter can be computed by a
calculation, of which it would be impossible to give an account in this
place. The mass of Jupiter, as determined by this method, agrees with
the mass obtained in a totally different manner from the satellites.

Nor have we yet exhausted the resources of astronomy in its bearing on
this question. We can discard the planetary system, and invite the
assistance of a comet which, flashing through the orbits of the planets,
occasionally experiences large and sometimes enormous disturbances. For
the present it suffices to remark, that on one or two occasions it has
happened that venturous comets have been near enough to Jupiter to be
much disturbed by his attraction, and then to proclaim in their altered
movements the magnitude of the mass which has affected them. The
satellites of Jupiter, the minor planets, and the comets, all tell the
weight of the giant orb; and, as they all concur in the result (at least
within extremely narrow limits), we cannot hesitate to conclude that the
mass of the greatest planet of our system has been determined with
accuracy.

The results of these measures must now be stated. They show, of course,
that Jupiter is vastly inferior to the sun--that, in fact, it would take
about 1,047 Jupiters, all rolled into one, to form a globe equal in