of twine and the hammer, and go up to the top
window, open it, and look out."

The boy did not stop to say "What for?" but ran up-stairs, opened the
window, and looked out, to find his uncle beneath with the long rod.

"Lower down the end of the string," he cried; and this was done, Tom
watching, and seeing it tied to the end of the rod where the brass nail
stuck through.

"Haul up, Tom."

The twine was tightened, and the end of the rod drawn up till Tom could
take it in his hand.

"Now take away the string."

This was done.

"Get your hammer."

"It's here on the window-sill, uncle."

"That's right. Now look here: I want you to lean out, and drive that
nail in between two of the bricks, so that this marking-point at my end
may hang just a few inches above the bottom of my piece of zinc. I'll
guide it. That's just right. Now drive in the nail."

"Must come an inch higher, so that the nail may be opposite a joint."

"Take it an inch higher, and drive it in."

This was done, and the rod swung like an immensely long wooden pendulum.

"That's right," cried Uncle Richard; "the nail and this point are
exactly twenty-four feet apart. Now keep your finger on the head of the
nail to steady it while I mark the zinc."

Tom obeyed, and looked down the while, to see his uncle move the rod to
and fro, till he had scored in the sheet of zinc a curve as neatly and
more truly than if it had been done with a pair of compasses.

"That's all, Tom," he said. "Take out the nail and lower the rod down
again carefully, or it will break."

All this was done, and Tom descended to find that both the rod and the
sheet of zinc had been carried in, the latter laid on the bench, and
displaying a curve deeply scratched upon it where the sharp-pointed
bradawl had been drawn.

"There, Tom," said Uncle Richard, "that curve is exactly the one we have
to make in our speculum, so that we may have a telescope of twelve feet
focus. Do you understand?"

"No," said Tom bluntly.

"Never mind--you soon will. It means that when we have ground out the
glass so that it is a hollow of that shape, all the light reflected will
meet at a point just twelve feet distant from its surface. Now we have
begun in real earnest."

He now took a keen-edged chisel, and pressing the corner down proceeded
to deepen the mark scored in the zinc with the greatest care, until he
had cut right through, forming the metal into two moulds, one of which
wa