quite true, as Helmholtz says, that Young was in advance of his
age; but something is to be added which illustrates the responsibility
of our public writers. For twenty years this man of genius was
quenched--hidden from the appreciative intellect of his
country-men--deemed in fact a dreamer, through the vigorous sarcasm of
a writer who had then possession of the public ear, and who in the
_Edinburgh Review_ poured ridicule upon Young and his speculations. To
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.

Sec. 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
motion of the wave itself, and the motion of the particles which at
any moment constitute the wave.

Stand upon the seashore and observe the advancing rollers before they
are distorted by the friction of the bottom. Every wave has a back and
a front, and, if you clearly seize the image of the moving wave, you
will see that every particle of water along t