of the cooking process. When they were
all safely stowed away in their places, Tom gave voice to the curiosity
with which he was almost bursting.

"Sam," he said, "how did you do that?"

"How did I do what, Tom?"

"How did you make the sun set the cotton on fire?"

"I don't know whether I can make you understand it or not," said Sam,
"but I'll try. You know light always goes in straight lines, if left to
itself, don't you?"

"No, I didn't know that!

"Yes you did, only you never thought of it. If you want to keep light
out of your eyes, you always put your hand between them and the light,
because you know the light goes straight and so will not go around your
hand."

"Yes, that's true, and when I want to make a shadow anywhere, I put
something right before the light."

"Certainly. Well, the rays of the sun all come to us straight, and side
by side. They are pretty hot, but not hot enough to set fire to anything
that way. But if you can gather a good many of these rays together and
make them all shine on one little spot, they will set fire to whatever
they fall on. Now a piece of glass or any other thing that you can see
through easily,--that is, any _transparent_ thing, lets the sunlight
through it, and if it is flat on both sides, it doesn't change the
directions of the rays. But if both sides are rounded out, or if one
side is rounded out and the other side is flat, it turns all the rays a
little, and brings them right together in a point not far from the
glass. If the sides are hollowed _in_ instead of bulging out, the rays
scatter, and if one side bulges out and the other bulges in, as they do
in a watch crystal, one side scatters and the other side collects the
rays, and so it is the same as if the glass had been perfectly flat, one
side undoes the other's work. Now I have no glass which bulges out on
both sides, and none that bulges out on one side and is flat on the
other, but my watch crystal bulges out on one side and in on the other.
But when I filled it with water, the water being as clear as the glass,
it made it flat on top and bulging underneath, and so it gathered the
sun's rays together in the light spot you saw, and set fire to the
cotton."

"Yes, but why did you have to wait till noon?" asked Tom.

"Because the glass must be held right across the rays of light, and as I
couldn't turn the crystal to either side without spilling the water, I
had to use it at noon, when the sun was almost ex