Mr Barlow, "is what they call a _lever_, and all the sticks
that you have been using to-day are only levers of a different
construction. By these short trials, you may conceive the prodigious
advantage which they are of to men; for thus can one man move a weight
which half-a-dozen could not be able to do with their hands alone; thus
may a little boy, like you, do more than the strongest man could effect
who did not know these secrets. As to that instrument by which you were
so surprised that Harry could cleave such a vast body of wood, it is
called a wedge, and is almost equally useful with the lever. The whole
force of it consists in its being gradually narrower and narrower, till
at last it ends in a thin edge, capable of penetrating the smallest
chink. By this we are enabled to overthrow the largest oaks, to cleave
their roots, almost as hard as iron itself, and even to split the solid
rocks." "All this," said Tommy, "is wonderful indeed; and I need not ask
the use of them, because I see it plainly in the experiments I have made
to-day."

"One thing more," added Mr Barlow, "as we are upon this subject, I will
show you." So he led them into the yard, to the bottom of his granary,
where stood a heavy sack of corn. "Now," said Mr Barlow, "if you are so
stout a fellow as you imagine, take up this sack of corn, and carry it
up the ladder into the granary." "That," replied Tommy, laughing, "is
impossible; and I doubt, sir, whether you could do it yourself."
"Well," said Mr Barlow, "we will, at least try what is to be done." He
then led them up into the granary, and, showing them a middle-sized
wheel, with a handle fixed upon it, desired the little boys to turn it
round. They began to turn it with some little difficulty, and Tommy
could hardly believe his eyes, when, presently after, he saw the sack of
corn, which he had despaired of moving, mounted up into the granary, and
safely landed upon the floor. "You see," said Mr Barlow, "here is
another ingenious contrivance, by which the weakest person may perform
the work of the strongest. This is called the _wheel_ and _axle_. You
see this wheel, which is not very large, turns round an axle which goes
into it, and is much smaller; and at every turn, the rope to which the
weight is fixed that you want to move, is twisted round the axle. Now,
just as much as the breadth of the whole wheel is greater than that of
the axle which it turns round, so much greater is the weight that th