“Well, centrifugal force throws it—helps throw it—out toward the edges, and—and—well, anyway, in a low there are places where the air is rising. And where it’s rising, it’s getting colder and the water-vapor it has in it has to come out as rain, and that’s why—well, more or less—why we get rain when a low comes along. And we know a low is coming when the pressure falls.”

The J. M. did not look, but he could feel Whitey’s grin.

“And what, children,” Sister Mary Rose was saying, “indicates the rise and fall of pressure?”

“The-barometer-sister,” said the class in unison, having obviously been well coached on that point.

“And always remember,” the J. M. went on in polite desperation, “that warm air is lighter, and so tends to rise over cold air.”

“Just like in a hot-air heating system,” said Sister Mary Rose.

“Yes, that’s right,” said the J. M. cordially, being glad of any assistance, “and our rain falls out of warm rising air that’s getting cooler.”

“Does the class have any questions?” asked Sister Mary Rose.

“What,” said a determined young voice, “is a typhoon?”

At this sign of interest and intelligence Sister Mary Rose beamed. Even the J. M. was encouraged.

“A typhoon,” he said, and then was suddenly overwhelmed by the impossibility of decently explaining a typhoon without the aid of integral calculus.

“A typhoon,” he repeated, “oh, that’s a big storm at sea they have over near the Philippine Islands.” To his amazement this answer satisfied everybody.

The class went out, each bobbing and thanking the J. M.

“That sure was a fine, scientific lecture—” said Whitey, “in terms of 1902. I thought you were trained in air-mass.”

But the J.M had done his duty, and was not going to be badgered. “Public service,” he said, feeling that he was learning some of the answers. But he thought to himself that the Chief ought to put someone like Whitey on jobs like that, not someone with a real scientific attitude. Then he settled to his desk and forgot all about it.

The storm now dominated a region which was as large as the United States westward from the Mississippi.

The Junior Meteorologist again looked at Maria with interest as for the third day she appeared on his map. At the time of observations she had centered, as closely as he could determine, at the one-hundred-eightieth meridian, almost in mid-ocean. Since this meridian was also the international Date Line, Maria had thus been in the anomalous position of having her warm front still in Thursday while her cold front stretched off through Friday for a while, until its trailing end was again in Thursday. In spite of her size Maria nowhere touched land. She centered exactly between Midway Island and the southernmost Aleutians, almost filling the broad space between.