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his delightful book, Gravitation , has re-
duced these investigations to simple geo-
metrical explanations.
In 1707 and 1713 Miraldi noticed that
the fourth satellite varies much in bright-
ness. W. Herschel found this variation to
depend upon its position in its orbit, and
concluded that in the positions of feeble-
ness it is always presenting to us a portion
522
