"It is a long hole made in the ground through a hill, that the train may go straight through the hill instead of going over it, for it is not easy for trains to go up and down hill." "Then it must be a very long hole indeed! When shall we get through it, and see daylight again?" "In a minute or two," replied she; "and this, no doubt, was the reason the whistle gave a signal, to warn any one, who might chance to be in the tunnel to get out of the way before the train went in. And this noise, disagreeable as it is, makes an excellent signal, it is heard at so great a distance; and it is so unlike all other sounds that it can never be mistaken." "And how can they make such a long hole, and so large too! big enough for the train to pass through?" "They begin," said his father, "by digging into the hill at both ends, the end at which we came in, and the end at which we shall soon go out, and they carry away the earth in wheelbarrows as they go on." "What a quantity of ground there must be to wheel away!" said Willy; "and are we very low under ground now?" "We have not gone down," replied his father; "but then the hill is high above our heads, much higher than a house." "Oh dear!" cried Willy, "if the hill were to fall upon us, would it not crush us to death?" "Certainly it would, but there is no danger of that. Were you ever afraid that the ceiling of a room would fall down, and kill you?" "Oh no," said Willy, laughing, "I never thought of such a thing." "Because it never happens," said his father; "nor does it happen to a hill to fall down into a tunnel. Builders, who understand how to build houses, and engineers, who know how to make tunnels, take care to construct them in such a manner that they shall not fall." "I think, Mamma," said Willy, "that we are getting near the end of the tunnel, for I see a little light." "That glimmering of light," said his father, "comes from a hole which has been made from the top of the hill, and is called a _shaft_. When a tunnel is very long, it is so difficult to get rid of the earth that it is found easier to take it away from the middle part of the tunnel, by pulling it up through a shaft, or well." "Then," said Willy, "they dig a shaft just as they dig a well, I suppose, only they bring up baskets full of earth instead of water. I remember seeing the well dug in our garden." "Then you must have observed, that when they first began to dig the well they brought up earth; it was not till they had dug down to a spring of water that they brought up water." "These shafts have also the advantage of letting light and air into the tunnel." "It is but very little," replied Willy, "for I could hardly have seen you and Mamma, if it had not been for the little lamp at the top of the carriage." "Now, Willy," said his mother, "I think you will soon see us by daylight, for we are coming to the end of the tunnel. Mind that you look at the sides of the tunnel before we leave it."