VIII
HOW THE PLANETS ARE WEIGHED
You ask me how the planets are weighed? I reply, on the same
principle by which a butcher weighs a ham in a spring-balance.
When he picks the ham up, he feels a pull of the ham towards the
earth. When he hangs it on the hook, this pull is transferred from
his hand to the spring of the balance. The stronger the pull, the
farther the spring is pulled down. What he reads on the scale is
the strength of the pull. You know that this pull is simply the
attraction of the earth on the ham. But, by a universal law of
force, the ham attracts the earth exactly as much as the earth
does the ham. So what the butcher really does is to find how much
or how strongly the ham attracts the earth, and he calls that pull
the weight of the ham. On the same principle, the astronomer finds
the weight of a body by finding how strong is its attractive pull
on some other body. If the butcher, with his spring-balance and a
ham, could fly to all the planets, one after the other, weigh the
ham on each, and come back to report the results to an astronomer,
the latter could immediately compute the weight of each planet of
known diameter, as compared with that of the earth.
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