nded by it of another of the natural phenomena, of the true
explanation of which I have not been able to satisfy myself. It is this:
what makes the earth freeze harder and deeper under a trodden path than
the untrodden earth around it? All that I have asked, say it is because
the trodden earth is more compact. But is that reason a sufficient one?"

8. "No," said Bunker, "but I will tell you what the reason is, for I
thought that out long ago. You know that, in the freezing months, much of
the warmth we get is given out by the earth, from which, at intervals, if
not constantly, to some extent, ascend the warm vapors to mingle with and
moderate the cold atmosphere above.

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 t