both.  This line would slope downwards from Newton to Young, because Newton was certainly the taller man of the two.  But the slope would not be steep, for the difference of stature was not excessive.  The line would form what engineers call a gentle gradient from Newton to Young.  Place underneath this line the biggest man born in the interval between both.  It may be doubted whether he would reach the line; for if he did he would be taller intellectually than Young, and there was probably none taller.  But I do not want you to rest on English estimates of Young; the German, Helmholtz, a kindred genius, thus speaks of him:  “His was one of the most profound minds that the world has ever seen; but he had the misfortune to be too much in advance of his age.  He excited the wonder of his contemporaries, who, however, were unable to follow him to the heights at which his daring intellect was accustomed to soar.  His most important ideas lay, therefore, buried and forgotten in the folios of the Royal Society, until a new generation gradually and painfully made the same discoveries, and proved the exactness of his assertions and the truth of his demonstrations.”

It is 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.