Thus, at the depth of 1000 miles, the
pressure on every cubic inch is more than 2000 tons, a weight
which would greatly condense the hardest metal.
We come now to the planets. I have said that the mass or weight of
a heavenly body is determined by its attraction on some other
body. There are two ways in which the attraction of a planet may
be measured. One is by its attraction on the planets next to it.
If these bodies did not attract one another at all, but only moved
under the influence of the sun, they would move in orbits having
the form of ellipses. They are found to move very nearly in such
orbits, only the actual path deviates from an ellipse, now in one
direction and then in another, and it slowly changes its position
from year to year. These deviations are due to the pull of the
other planets, and by measuring the deviations we can determine
the amount of the pull, and hence the mass of the planet.
The reader will readily understand that the mathematical processes
necessary to get a result in this way must be very delicate and
complicated. A much simpler method can be used in the case of
those planets which have satellites revolving round them, because
the attraction of the planet can be determined by the motions of
the satellite.
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