"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 again, why could it not keep on doing that all the time, until the balance was put straight again?" "It would," the scientist agreed, "but for one thing you have forgotten." "And what's that?" "The rotation of the earth." A single drop of rain fell, then another, making a splash as large as a twenty-five cent piece. "Now see it come!" said the scientist. As though his words had summoned it, a liquid opacity, like a piece of clouded glass, thrust itself between their eyes and the landscape. So suddenly it came that Stuart actually did not realize that this was falling rain, until, looking at the ground, he saw the earth dissolve into mud before his eyes and saw the garden turn into what seemed like the bed of a shallow river. The wind whistled with a vicious note. The squall lasted scarcely a minute, and was gone.