Part 22 (1/2)
”Begin to polish the glass, uncle?”
”Not yet Fetch those two pieces of lath” Tom fetched a couple of thin pieces of wood, each a little over twelve feet long These were laid upon the bench and screwed together, so as to
Then at one end a hole was e brass-headed nail was thrust, while through the other end a sharp-pointed bradaas bored, so as to leave its sharp point sticking out a quarter of an inch on the other side
”So far so good,” said Uncle Richard ”Do you knoe are going to do, Tom?” Tom shook his head
”Strike the curve on that piece of zinc that we are to make our speculum”
”Curve?” said Toe is, but we are going to work at the face”
”But arn't you going to polish it into a looking-glass?”
”Yes; but not a flat one--a plane That would be of no use to us, Tom; we must have a parabolic curve”
”Oh,” said Tom, who only knew parabolas froh an old Greek friend called Euclid
”Be patient, and you'll soon understand,” continued Uncle Richard, who proceeded to secure the sheet of zinc to a piece of board byit out, and fixing the board just at the botto nails at the sides of the board were sufficient, and then he led the way in
”Now, Too up to the top , open it, and look out”
The boy did not stop to say ”What for?” but ran up-stairs, opened the , 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, To it tied to the end of the rod where the brass nail stuck through
”Haul up, Tohtened, and the end of the rod drawn up till To”
This was done
”Get your hammer”
”It's here on the -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 thisjust a few inches above the bottoht Now drive in the nail”
”Must coher, so that the nail her, and drive it in”
This was done, and the rod swung like an iht,” 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, Toain carefully, or it will break”