"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."