ill be as flat as flat," said Tom to himself, "but I don't like to
tell him so."

"There, that will do," said Uncle Richard, at the end of ten minutes.
"Now then, are the pieces both flat?"

"No, uncle; the bottom piece is rounded and the top hollowed, but I
can't see why."

"Then I'll tell you: because the centre gets rubbed more than the sides,
Tom. There, take paper and salt back, and we'll begin."

Tom caught up the paper, and soon returned, eager to commence; and after
a little instruction as to how he was to place his hands upon the top
glass, Uncle Richard placed himself exactly opposite to his nephew, with
the upturned cask between them.

"Now, Tom, it will be a very long and tedious task with this great
speculum; hot work for us too, so we must do a bit now and a bit then,
so as not to weary ourselves out. Ready?"

"Yes, uncle."

"Then off."

"It will be a tiresome job," thought Tom, as, trying hard to get into
regular swing with his uncle, the top glass was pushed to and fro from
one to the other; but at each thrust Uncle Richard made a half step to
his left, Tom, according to instructions, the same, so that the glass
might be ground regularly all over. At the end of a quarter of an hour
it was slid on one side, and more water and sand applied. Then on
again, and the grinding continued, the weight of the glass making the
task very difficult. But Tom worked manfully, encouraged by his uncle's
assurance that every day he would grow more accustomed to the work, and
after two more stoppages there was a cessation.

"There!" cried Uncle Richard; "one hour's enough for the first day. It
wants faith to go on with such a business, Tom."

As he spoke the future speculum was carefully lifted off the lower one,
sponged with clean water, and on examination proved to be pretty well
scratched in the middle in a round patch, but the marks grew less and
less, till at the edge of the glass it was hardly scratched at all.

"There, you see where we bite hardest," said Uncle Richard; "now we'll
give it a rest, and ourselves too."

"But we shall never get done like this," cried Tom.

"Oh yes, we shall, boy; and I'm not going to leave off our work. Let's
see: this we must call the workshop, the floor above our laboratory, and
the top of course the observatory. Now then, let's go up into our
laboratory, and I'll give you a lesson in elutriation."

CHAPTER THIRTEEN.

"I haven't got a dictionary here, unc