"Begin to polish the gla.s.s, 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 make one rod just over twenty-four feet long.

Then at one end a hole was made, into which a large bra.s.s-headed nail was thrust, while through the other end a sharp-pointed bradawl was 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 know what we 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 Tom; "why, it's quite round now."

"Yes; the edge is, but we are going to work at the face."

"But arn't you going to polish it into a looking-gla.s.s?"

"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 from a cursory acquaintance with them through 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 by means of four tacks at its corners, and ended by carrying it out, and fixing the board just at the bottom of the border, close to the window.

A couple of strong nails at the sides of the board were sufficient, and then he led the way in.

"Now, Tom, take that ball 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 bra.s.s 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 compa.s.ses.

"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 gla.s.s 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 was to gauge the lower disc, the other the upper. The edges of these were then rubbed carefully together as they lay flat upon the bench, till their edges were quite smooth; then some of the unnecessary zinc was cut away, a couple of big holes punched in them, and they were hung upon a couple of nails over the bench ready for use.

"Next thing," cried Uncle Richard, "is to begin upon the speculum itself, so now for our apparatus. Here we have it all: a bowl of fine sifted silver sand, a bucket of water, and a sponge. Very simple things for bringing the moon so near, eh?"

"But is that all we want, uncle?"

"At present, my boy," said Uncle Richard, proceeding to wet some of the sand and pretty well cover the disc of gla.s.s fixed upon the cask-head.

"That's for grinding, as you see."

"Yes, uncle; but what are you going to rub it with?"

"The other disc. Here, catch hold. Be careful."

Tom obeyed, and the smooth piece of plate-gla.s.s was laid flat upon the first piece, crushing down the wet sand, and fitting well into its place.

"Now, my boy, if we rub those two together, what will be the effect?"

"Grind the gla.s.s," said Tom. "I once made a transparent slate like that, by rubbing a piece of gla.s.s on a stone with some sand and water.

But I thought you wanted to hollow out the gla.s.s?"

"So I do, Tom."

"But that will only keep the pieces flat."

"I beg your pardon, my boy. If we rub and grind them as I propose, one of the discs will be rounded and the other hollowed exactly as I wish."

Tom stared, for this was to his way of thinking impossible.

"Are you sure you are right, uncle? Because if you are not, it would be so much trouble for nothing."

"Let's prove it," said Uncle Richard, smiling. "Go to the kitchen door, and ask the cook for a couple of good-sized pieces of salt and the meat-saw."

The cook stared, but furnished the required pieces, which were soon shaped into flat slabs with the saw. Then a sheet of newspaper was spread, and one of the flat pieces of salt placed upon the other.

"There you are, Tom," said his uncle. "I want you to see for yourself; then you will work better. Now then, grind away, keeping the bottom piece firm, and the top going in circular strokes, the top pa.s.sing half off the bottom every time."