Barbican attempted in vain to obtain even a tolerable approximation of
the velocity at which the Projectile was now moving. He had to content
himself with the knowledge that it was diminishing at a uniform rate--of
which indeed a little reflection on a well known law of Dynamics readily
convinced him. He had not much difficulty even in explaining the matter
to his friends.
"Once admitting," said he, "the Projectile to describe an orbit round
the Moon, that orbit must of necessity be an ellipse. Every moving body
circulating regularly around another, describes an ellipse. Science has
proved this incontestably. The satellites describe ellipses around the
planets, the planets around the Sun, the Sun himself describes an
ellipse around the unknown star that serves as a pivot for our whole
solar system. How can our Baltimore Gun Club Projectile then escape the
universal law?
"Now what is the consequence of this law? If the orbit were a _circle_,
the satellite would always preserve the same distance from its primary,
and its velocity should therefore be constant. But the orbit being an
_ellipse_, and the attracting body always occupying one of the foci, the
satellite must evidently lie nearer to this focus in one part of its
orbit than in another.
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