the celebrated Frenchmen Fresnel and Arago he was first indebted for
the restitution of his rights; for they, especially Fresnel,
independently remade and vastly extended his discoveries. To the
students of his works Young has long since appeared in his true light,
but these twenty blank years pushed him from the public mind, which
became in time filled with the fame of Young's colleague at the Royal
Institution, Davy, and afterwards with the fame of Faraday. Carlyle
refers to a remark of Novalis, that a man's self-trust is enormously
increased the moment he finds that others believe in him. If the
opposite remark be true--if it be a fact that public disbelief weakens
a man's force--there is no calculating the amount of damage these
twenty years of neglect may have done to Young's productiveness as an
investigator. It remains to be stated that his assailant was Mr. Henry
Brougham, afterwards Lord Chancellor of England.


§ 4. _Wave-Motion, Interference of Waves, 'Whirlpool Rapids' of
Niagara_.

Our hardest work is now before us. But the capacity for hard work
depends in a great measure on the antecedent winding up of the will; I
would call upon you, therefore, to gird up your loins for coming
labours.

In the earliest writings of the ancients we find the notion that sound
is conveyed by the air. Aristotle gives expression to this notion, and
the great architect Vitruvius compares the waves of sound to waves of
water. But the real mechanism of wave-motion was hidden from the
ancients, and indeed was not made clear until the time of Newton. The
central difficulty of the subject was, to distinguish between the