ke, Mary, but the air has been proved to be a
fluid. Very well; no fluid in large bodies moves with an equal velocity
throughout. Part of it is rapid and part quite stagnant. The stagnant
places of the air produce this green scum, this mossy, unwholesome, and
injurious stuff; while the overrapid motion causes this iron appearance,
this hard surface, and general sterility. By the simplest of simple
contrivances, I make this evil its own remedy. An equable impulse given
to the air produces an adequate uniform flow, preventing stagnation in
one place, and excessive vehemence in another. And the beauty of it is
that by my new invention I make the air itself correct and regulate its
own inequalities."

"How clever you are, to be sure!" exclaimed Mary, wondering that her
father could not see it. "Oh, Willie, you will make your fortune by it!
However do you do it?"

"The simplicity of it is such that even you can understand it. All
great discoveries are simple. I fix in a prominent situation a large
and vertically revolving fan, of a light and vibrating substance. The
movement of the air causes this to rotate by the mere force of the
impact. The rotation and the vibration of the fan convert an irregular
impulse into a steady and equable undulation; and such is the elasticity
of the fluid called, in popular language, 'the air,' that for miles
around the rotation of this fan regulates the circulation, modifies
extremes, annihilates sterility, and makes it quite impossible for moss
and green scum and all this sour growth to live. Even you can see, Mary,
how beautiful it is."

"Yes, that I can," she answered, simply, as they turned the corner upon
a large windmill, with arms revolving merrily; "but, Willie dear, would
not Farmer Topping's mill, perpetually going as it is, answer the same
purpose? And yet the moss seems to be as thick as ever here, and the
ground as naked."

"Tush!" cried Willie. "Stuff and nonsense! When will you girls
understand? Good-by! I will throw away no more time on you."

Without stopping to finish his sentence he was off and out of sight
both of the mill and Mary, before the poor girl, who had not the least
intention of offending him, could even beg his pardon, or say how much
she wanted him; for she had not dared as yet to tell him what was the
purpose of her walk, his nature being such that no one, not even his own
mother, could tell what conclusion he might come to upon any practical
question. He