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 will 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 gla.s.s, 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 gla.s.s 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 gla.s.s 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 gla.s.s making the task very difficult. But Tom worked manfully, encouraged by his uncle's a.s.surance 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 gla.s.s 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, uncle," said Tom, with a smile, as they stood at the ma.s.sive table under the window in the laboratory. "I don't know what elutriation means."

"I dare say not. I didn't till I was nearly fifty, Tom, but you soon shall know. Fetch that tin off the shelf."

Tom obeyed, and found a label on the top, on which was printed "Best Ground Emery."

"Well, you know what that is?"

"Emery? Powdered gla.s.s," said Tom promptly.

"Wrong. Diamond cuts diamond, Tom, but we want something stronger than powdered gla.s.s to polish itself. Emery is a mineral similar in nature to sapphire and ruby, but they are bright crystals, and emery is found in dull blocks."

"Then it's very valuable?" said Tom.

"Oh, no. It is fairly plentiful in Nature, and much used. Now then, we want coa.r.s.e emery to grind our speculum after we have done with the sand, and then different degrees to follow, till we get some exquisitely fine for polishing. How are we to divide the contents of that tin so as to graduate our grinding and polishing powder?"

"Sift it, of course, uncle."

"And where would you get sieves sufficiently fine at last?"

"Muslin?"

"Oh, no. Here is where elutriation comes in, Tom; and here you see the use of some of the things I brought back from London the other day. To work. Bring forward that great pan."

This was done.

"Now empty in the contents of this packet."

Tom took up a little white paper of something soft, opened it, and poured the contents into the pan.

"Powdered gum arabic?" he said.

"Yes. Now empty the tin of emery upon it."

Tom opened the tin, and found within a dark chocolate-looking powder, which felt very gritty between his finger and thumb. This he emptied upon the gum arabic, and, in obedience to instructions, thoroughly mixed both together.

"To make the fine emery remain longer in suspension," said his uncle, "keep on stirring, Tom."

"All right, uncle. What, are you going to pour water in? It's like making a Christmas pudding."

For Uncle Richard took up a can of water, and began to pour a little in as Tom stirred, changing the powder first into a paste, then into a thick mud, then into a thin brown batter, and at last, when a couple of gallons or so had been poured in and the whole well mixed, the great pan was full of a dirty liquid, upon the top of which a sc.u.m gathered as the movement ceased. This sc.u.m Uncle Richard proceeded to skim off till the surface was quite clear, and then he glanced at his watch.

"Is that sc.u.m the elutriation?" said Tom, with a faint grin.

"No, boy, the impurity; throw it down the sink. Now, Tom, we want to get our finest polishing emery out of that mixture, and it will take an hour to form--sixty-minute emery, the opticians call it; so while it is preparing, we'll go and have another turn at the speculum."

They descended, leaving the pan standing on the heavy table, and after spreading wet sand upon the lower disc of gla.s.s, the loose one was once more set in motion, and uncle and nephew, with quarter-hour rests for examination and wetting the surfaces, patiently ground away for an hour, by which time, upon the speculum being sponged, it was found that the greater part of the upper gla.s.s was deeply scratched.

"This is going to be an awfully long job," thought Tom.

"Yes, it is," said his uncle, who aptly read his thoughts, "a very long job, Tom; but good things have to be worked for, boy."

"Oh, I'm not going to be tired, uncle. It's like working for a grand prize."

"It is. Now then, let's see to the emery. Our finest must be ready by now. Now I want all the water, from which the emery has settled down to the bottom, drawn off into that great white basin. How is it to be done?"

"Pour it off," said Tom.

"No; couldn't be done without disturbing the bottom. Let's try syphoning."

Uncle Richard placed the basin upon a stool below the level of the table, took up a gla.s.s tube bent somewhat in the shape of a long-shanked hook, placed the short end gently beneath the surface of the nearly clear water, his lips to the long end, drew out the air, and the water followed directly from the atmospheric pressure, and ran swiftly into the basin.