pose, 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 b