9.  “Now these ascending streams of warm air would be almost wholly obstructed by the compactness of a trodden path, and they would naturally divide at some distance below it, and pass up through the loose earth on each side, leaving the ground along the line of the path, to a great depth beneath it, a cold, dead mass, through which the frost would continue to penetrate, unchecked by the internal heat, which, in its unobstructed ascent on each side, would be continually checking or overcoming the frost in its action on the earth around.

10.  “That, sir, is the true philosophy of the case, you may depend upon it.  But we will now drop the discussion of these matters; for I am abundantly satisfied that you have not only knowledge enough, but that you can think for yourself.  And now, sir, all I wish to know further about you is, whether you can teach others to think, which is half the battle with a teacher.  But as I have had an eye on this point, while attending to the others, probably one experiment, which I will ask you to make on one of the boys here, will be all I shall want.”

“Proceed, sir,” said the other.

11.  “Ay, sir,” rejoined Bunker, turning to the open fireplace, in which the burning wood was sending up a column of smoke, “there, you see that smoke rising, don’t you?  Well, you and I know the, reason why smoke goes upward, but my youngest boy does not, I think.  Now take your own way, and see if you can make him understand it.”

12.  Locke, after a moment’s reflection, and a glance round the room for something to serve for apparatus, took from a shelf, where he had espied a number of articles, the smallest of a set of cast-iron cart boxes, as are usually termed the round hollow tubes in which the axletree of a carriage turns.  Then selecting a tin cup that would just take in the box, and turning into the cup as much water as he judged, with the box, would fill it, he presented them separately to the boy, and said,

“There, my lad, tell me which of these is the heavier.”

13.  “Why, the cart box, to be sure,” replied the boy, taking the cup, half-filled with water, in one hand, and the hollow iron in the other.

“Then you think this iron is heavier than as much water as would fill the place of it, do you?” resumed Locke.

“Why, yes, as heavy again, and more too—­I know it is,” promptly said the boy.

14.  “Well, sir, now mark what I do,” proceeded the former, dropping into the cup the iron box, through the hollow of which the water instantly rose to the brim of the vessel.

“There, you saw that water rise to the top of the cup, did you?”

“Yes, I did.”

“Very well, what caused it to do so?”

15.  “Why, I know well enough, if I could only think:  why, it is because the iron is the heavier, and as it comes all around the water so it can’t get away sideways, it is forced up.”

“That is right; and now I want you to tell what makes that smoke rise up the chimney.”