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"T HE 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."
"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 the rounds of a ladder. The red liquid goes up or down from division to division just as we mount or descend a flight of stairs step by step. If it grows warmer, the red liquid moves and little by little climbs the steps; if colder, it goes down the ladder. Thus the heat can be estimated according to the step or degree where the liquid stops.
"It is freezing when the liquid goes down to zero; the heat is that of boiling water when it goes up to division 100. The intermediate steps or degrees indicate, evidently, other states of heat, greater when the degree is higher up on the ladder.
"The degree of heat of any body, as indicated by the thermometer, is called its temperature. Thus we say the temperature of freezing water is zero, that of boiling water one hundred degrees."
"One morning," said Emile, "when you sent me to get something from your room, I put my hand on the little bulb of the thermometer. The red liquid began to go up, little by little."
"It was the warmth of your hand that made it go up."
"I wanted to see how high the liquid would go, but I had not patience to wait till the end."
"I will tell you. At last the thermometer would have marked at most 38 degrees, which is the temperature of the human body."
"And in the very hot days of summer what degree does the thermometer mark?" asked Jules.
"In our region the greatest heat of summer is from 25 to 35 degrees."
"And in the hottest countries of the world?" Claire inquired.
"In the hottest countries, Senegal, for example, the temperature rises to 45 and 50 degrees. It is twice as hot as our summer."