s 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 glass 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-glass 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 glass," said Tom. "I once made a transparent slate like
that, by rubbing a piece of glass on a stone with some sand and water.
But I thought you wanted to hollow out the glass?"

"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 passing half
off the bottom every time."

Tom began, and worked away, while from time to time the lower piece was
turned round.

"Nice fine salt," said Uncle Richard; "cook ought to be much obliged."

"It w