Speaking more plainly, genius is the enthusiasm for self-improvement; it ceases or sleeps the moment it desists from seeking some object which it believes of value, and by that object it insensibly connects its self-improvement with the positive advance of the world.
At present Lenny's genius had no bias that was not to the Positive and Useful.

It took the direction natural to its sphere, and the wants therein,--namely, to the arts which we call mechanical.

He wanted to know about steam-engines and Artesian wells; and to know about them it was necessary to know something of mechanics and hydrostatics; so he bought popular elementary works on those mystic sciences, and set all the powers of his mind at work on experiments.
Noble and generous spirits are ye, who, with small care for fame, and little reward from pelf, have opened to the intellects of the poor the portals of wisdom! I honour and revere ye; only do not think ye have done all that is needful.

Consider, I pray ye, whether so good a choice from the tinker's bag would have been made by a boy whom religion had not scared from the Pestilent, and genius had not led to the self-improving.

And Lenny did not wholly escape from the mephitic portions of the motley elements from which his awakening mind drew its nurture.