“If they are polished?” asked Rollo.
“Whether they are polished or not,” replied his father. “I do not know that that makes any difference. Because, you see, in being conducted, the heat moves through the middle of the body, and not along its surface so much; and I don’t know that the character of the surface is of much importance. Some metals conduct better than others. The philosophers have a curious kind of an instrument to show the different conducting powers of different substances.”
“What is it, father?” said Rollo.
“Why, they have a small iron plate——”
“Round?” asked Rollo.
“No, square,” replied his father.
“I thought plates were always round,” said Rollo.
“No,” replied his father; “a plate, in philosophical language, is any flat and comparatively thin piece of metal, or other such substance, whatever its shape may be. This plate that I was speaking of is square, or oblong,—I believe it is oblong,—and it is about as large as my hand, and perhaps half an inch thick.
“In this plate,” continued his father, “there are drilled a number of little holes.”
“How many, sir?” asked Rollo.