ed. Tell the cook to pour a pail of
water on the kitchen fire before she leaves. See, yourself, that every
place is securely fastened. The rain will be here in ten minutes."

The negro, who was gray with fright, flashed a quick look of relief at
the orders to seek the hurricane wing, and ran off at full speed.

"The first rain-squalls will not be bad," continued the "Old Doc," "and
I like to stay out as long as I can, to watch its coming. It will be
nearly dark when this one strikes us, though, and there won't be much to
see."

"But what starts them, sir?" queried the boy, who had become intensely
interested, since the grim phantasmagoria was unfolding itself on sea
and sky before his eyes.

"As I have told you, it is the creation of a super-heated and saturated
mass of air, only possible in a calm region, such as the Caribbean west
of the West Indies, or the doldrum region southeast of them. Let me show
you how it happens.

"A region of air, over a tropical sea, little moved by wind-currents,
becomes warmer than the surrounding region of air; the air over this
region becomes lighter; the lighter air rises and flows over the colder
layers of surrounding air, increasing the pressure on that ring and
increasing the inward flow to the warm central area where the air
pressure has been diminished by the overflow aloft. The overflowing air
reaches a point on the outside of the cold air area, when it again
descends, and once more flows inward to the center, making a complete
circuit. Do you understand so far?"

Stuart knitted his brows in perplexity.

"I--I think so, but I'm not sure," he said. "Then the barometer rose,
yesterday, because we were in the cold air area, which became heavier
because there was a layer of warm air on the top of it. The storm has
moved westward. The cold air section has passed. The barometer is
falling now because we're in the region of warm air, which is steadily
rising and is therefore lighter. That shows we're nearer to the center.
Is that it?"

The scientist tapped his fingers on the arm of his chair in pleased
appreciation.

"Very good," he said, "you are exactly right. And, from now on, the
barometer will drop suddenly, for the whirl of the wind will make a
partial vacuum in the very center of the hurricane."

"But I don't see what makes it whirl," protested the boy. "If it goes up
in the middle, flows over at the top and comes down at the outside and
then flows into the middle ag