“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.