"You bet the sun's hot," the boy agreed, "but Mr. Levin told me the other day that we only get a two-billionth part of the heat put out by the sun. Did you know that, Ross? The sun has heat enough to warm two billion Earths as big as this one. Even at that, Dan'l, the amount of heat we get from the sun would make thirty-seven billion tons of freezing water boil in one minute."

The negro's jaw dropped.

"Yo' not fooling?" he said.

"Not a bit."

"Ah's hot," he said. "Ah's goin' to boil, soon."

"Cheer up, Dan'l. You'll cool off tonight," suggested the older lad.

"Nearly everything that takes in heat has to give it out again. The earth, the sea and the dust in the air, all gradually let out some of the heat during the night. If it wasn't for that, everything would stay at the same temperature all night long. That's why it's always colder an hour before dawn than an hour after sunset.

"See, Dan'l, the earth and the air which take in heat easily and give off heat easily, by the end of the night, have got rid of a lot of their heat. At sea, though, where the water lets go its heat less easily, it is never as cold as on land. The thermometer shows when it's hot and when it's cold."

"Ah don't hold with none o' them fermometers," the old darky repeated.

"That's because you don't understand them," the crippled lad replied.

"It's dead easy, though. You see, Dan'l, when a thing is hot it gets bigger and when it's cold it gets smaller, that is, most things do."

"Ah don't see that, nohow," the negro answered. "A red hot stove is just 'zackly the same size as when the fire's out."

"No, it isn't, as a matter of fact," the lad replied, "but you can't always see the difference. Iron does get bigger as it gets hot. You've seen the steel rails on railroad tracks, haven't you, Dan'l."

"Sho'."

"Did you ever notice that there's a little crack between each rail? In winter, the crack is quite wide."

The negro thought for a moment.

"Is that the crack that makes a train b.u.mp?"

"Yes, that's it. Now, Dan'l, on a hot day in summer, you can't see any crack there at all, the rail has expanded or got bigger, and filled it up. On a frosty day in winter, there's a big crack, so big that you could drop a lead pencil between the ends of the rails. That's the difference of expansion on a steel rail between winter and summer."

"That's powehful little!"

"It's quite a good deal. I'll show you. Suppose, Dan'l, you had a small rubber ball filled with ink and there was a pipe out of the ball sticking straight up in the air, and suppose you put that little rubber ball in the crack between the rails."

"Yes?"

"Then, on a cold day, the rubber ball would have room enough. It wouldn't be squeezed and all the ink would stay in it. On a hot day, as the end of rails came together, they would squeeze the ball and the ink would squirt up. As there wouldn't be anywhere for it to go except through the tube, it would shoot up the tube, wouldn't it?"

"Sho'."

"So that you could tell, by the height of the ink in the tube, how much the rails had come closer together, or expanded. As the only thing that would make them expand would be the heat, you could measure the heat that way, couldn't you?"

"Ah reckon yo' could."

"That's what a thermometer does, Dan'l. The little bulb at the bottom contains something that's easily swelled by the heat. In a hot climate, quicksilver is used, because it doesn't boil except at a heat much greater than the air ever gets, though it freezes easily; in a cold climate, they use alcohol because it doesn't freeze except at a degree of cold much colder than the atmosphere ever gets, though it boils easily."

"Yo' fermometer's got blood in it!"

"No, the alcohol is colored, so that you can see it easily, Dan'l, that's all. The quicksilver, or the alcohol, is put into a little bulb and up from this bulb there runs a tube. That tube is awfully thin, sometimes a hundred times thinner than a hair. When a tube is as thin as that, even a tiny amount of expansion or contraction will make the quicksilver run up the tube or down. If you watch that thermometer I've got in that white shelter over there, Dan'l, you can easily tell when it's hotter and colder. It's nearly always hotter around noon."

"It's sho' mighty near noon now," Dan'l declared.

"How do you know?"

"Ah can tell that fo' sho', yas, suh!"

"How, Dan'l?"

"By mah own fermometer, Mist' Ross, an' that's mah inside. Right about five minutes befo' noon, thar's a little knock that says 'Tap, tap,'

Dan'l, yo're hungry.' An' that knockin's always right, Mistah Ross. Ah sho' is hungry right at that hyar time."

"It hasn't knocked yet, Dan'l, has it?"

The darky looked thoughtful.

"Ah hasn't felt it," he answered, "but Ah's got a feelin' that Ah can expect it now 'most any minute."

"Well," the younger lad answered, watching the black shadow of the pole as it stretched along the ground almost to his feet, "we'll find out how near right your inside is."

He took a piece of steel tape from his pocket and handed it to his chum.

"How long is it, Ross?" he asked. He bent down eagerly and watched the measuring of the shadow.

"Four feet, six inches," the older lad announced.

The negro looked at the shadow a moment and then burst into a hearty laugh.

"What is it, Dan'l?"

"Why, Mistah Ross, it ain't no use for yo' to measure that! Yo' done forgot that a shadder don't stay still."

"Why not?"

"A shadder keeps movin' round. Yo' ought to have thought o' that," he added seriously.

"We thought of it, all right, Dan'l," Anton answered. "See, the line of the shadow's already on one side of the tape. Try it again, Ross!"

"Four feet, five inches and three quarters," came the reply.

"What fo' makes that shorter?" queried the negro.

"Dan'l," said the younger boy, reprovingly, "why don't you use that thick head of yours a little? When you get up in the morning, isn't your shadow longer than it is in the middle of the day?"

"Sho', it stretches away off yonder!"

"And in the evening?"