“THE story of the gunner,” Jules remarked, “ended very differently from what one expected at the beginning. Just when one thinks the two travelers are done for, it turns out nothing more serious is in question than the roasting of two fowls. A shiver of fear seizes you when the man climbs the ladder with the cutlass between his teeth; the next minute you are laughing. That is a very amusing story; but it has turned us aside from the earthquakes. You have not told us yet the cause of these terrible movements of the ground.”

“If that interests you,” replied his uncle, “let us talk about it a little. I will tell you first that the farther you descend into the earth, the hotter it becomes. Excavations made by man for obtaining various minerals give us valuable information on this subject. The deeper they go, the hotter it is. For every thirty meters of depth there is an increase of one degree in temperature.”

“I don’t know very well what a degree is,” said Jules.

“And I don’t know anything about it,” confessed Emile.

“Let us begin with that; if not, it would be impossible for you to understand. In my room you have seen, on a little wooden board, a glass rod pierced by a very fine canal and ending at the bottom in a little bulb. In the bulb is a red liquid, which ascends or descends in the canal of the tube according to whether it is warmer or colder. That is called a thermometer. In freezing water the red liquid goes down to a point in the tube called zero; in boiling water it goes up to a point marked 100. The distance between these two points is divided into one hundred equal parts called degrees.”[3]

“Why degrees?” asked Emile.

“By that it is meant that these divisions have a certain resemblance to the degrees or steps of a flight of stairs, or t............