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similar behaviour of other comets. It must
be noted, however, that comets have a sen-
sible size, that all their parts cannot travel
in exactly the same orbit under the sun’s
gravitation, and that their mass is not suffi-
cient to retain the parts together very forcibly;
also that the inevitable collision of particles,
or else fluid friction, is absorbing energy,
and so reducing the comet’s velocity.
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