thermometer is a glass tube, fastened to a piece of wood or perhaps
tin, and inside is a thin, shiny column. This column is mercury, or
quicksilver. Some thermometers have, instead of mercury, alcohol,
colored red, so it can easily be seen.

You see mercury, or alcohol, will not freeze, except in much colder
weather than you ever have where you live, unless you live at the
North Pole. Up there it gets so cold that sometimes alcohol will
became as thick as molasses, and then it is not of any use in a
thermometer. But mercury will not freeze, even at the North Pole.

The word thermometer means something by which heat can be measured.
"Thermos" is a Greek word, meaning heat, and "Meter" means to measure.
Though of course a thermometer will measure cold as well as heat.

"Is it cold enough?" asked Hal, as Daddy Blake came back from looking
at the thermometer.

"Not quite," his father answered. "But the mercury is going down the
tube."

"What makes it go down?" asked Mab.

"Well, let me think a minute, and I'll see if I can make it simple
enough so you can understand," said Daddy Blake.

Those of you who have read the other "Daddy" books know how many
things Mr. Blake told his children, and what good times Hal and Mab
had with him. He was always taking them somewhere, and often one or
the other of the children would call out: