and science were never separated in his work; and both
were not unfrequently subservient to some fanciful caprice, some bizarre
freak of originality. Curiosity and love of the uncommon ruled his nature.
By intuition and by persistent interrogation of nature he penetrated many
secrets of science; but he was contented with the acquisition of
knowledge. Once found, he had but little care to distribute the results of
his investigations; at most he sought to use them for purposes of
practical utility.[237] Even in childhood he is said to have perplexed
his teachers by propounding arithmetical problems. In his maturity he
carried anatomy further than Delia Torre; he invented machinery for
water-mills and aqueducts; he devised engines of war, discovered the
secret of conical rifle-bullets, adapted paddle-wheels to boats, projected
new systems of siege artillery, investigated the principles of optics,
designed buildings, made plans for piercing mountains, raising churches,
connecting rivers, draining marshes, clearing harbours.[238] There was no
branch of study whereby nature through the effort of the inquisitive
intellect might be subordinated to the use of man, of which he was not
master. Nor, richly gifted as was Lionardo, did he trust his natural
facility. His patience was no less marvellous than the quickness of his
insight. He lived to illustrate the definition of genius as the capacity
for taking infinite pains.

While he was a boy, says Vasari, Lionardo modelled in terra-cotta certain
heads of women smiling. This was in the workshop of Verocchio, who had
already fixed a smile on David's face in bronze. When an old man, he left
"Mona Lisa" on the easel not quite finished, the portrait of a subtle,
shadowy, uncertain smile. This smile, this enigmatic revelation of a
movement in the soul, this seductive ripple on the surface of the human
personality, was to Lionardo a symbol of the secret of the world, an image
of the universal mystery. It haunted him all through his life, and
innumerable were the attempts he made to render by external form the magic
of this fugitive and evanescent charm.

Through long days he would follow up and down the streets of Florence or
of Milan beautiful unknown faces, learning them by heart, interpreting
their changes of expression, reading the thoughts through the features.
These he afterwards committed to paper. We possess many such sketches--a
series of ideal portraits, containing each an unso