"That made no difference whatever in the amount of heat in it," answered the Doctor. "When water boils at all it is just as hot as fire can make it, unless it is shut completely off from contact with the air, as is the case in steamboilers. You can't make it any hotter no matter how much you may 'fire up' under the kettle."
"Why, how's that?" asked "Little Tom," becoming interested. "The more fire you make in a stove the hotter the stove gets, and the hotter the room gets, too. Why isn't it the same way with a kettle of water?"
"I'll explain that," said the Doctor, "and I think I can make you understand it. When water boils it gives off the vapor which we commonly call steam. That is to say, some of the water is converted by heat into vapor. It requires a great deal of heat to make the change from liquid to vapor and so the process of giving off steam cools the water. That is why you put a lid on a pot that you wish to boil quickly. You do it to check the cooling process by confining the vapor and preventing a too rapid conversion of water into steam."
"Is that the reason that you can hold your hand in the steam from a kettle when you can't hold it in the water that the steam comes from," asked Jim.
"Yes. The steam is really hotter than the water, but it needs all its own heat to keep it in the form of vapor, and so it doesn't give off enough heat to burn your hand after it gets a little way from the pot and begins to expand freely. Now as I was saying the harder you boil water the more steam it gives off and the heating and cooling processes are so exactly balanced that boiling water stands always at a uniform temperature no matter whether it is boiling hard as we say, or only just barely boiling. But in a dense atmosphere it requires more heat to boil water than it does in a rarefied atmosphere like that up here on the mountain. At Leadville and other places lying from 10,000 to 14,000 feet above sea level in the Rocky mountains you can't boil potatoes at all and it takes full ten minutes to boil an egg into that condition which we call 'soft.' It all depends upon the temperature of boiling water, and that is considerably lower here than down in the valleys where we live."
"But Doctor," said Harry, "you promised to tell us how you find out how high we are above the sea level."
The Doctor got up, went to a tree and took down a scientific instrument.
"This," he said, "is an aneroid barometer. It measures the atmospheric pressure, and as that pressure steadily and pretty uniformly decreases as we go higher up, the instrument tells us at once how high we are."
"But will it measure so accurately that you can trust it?" asked one of the now eagerly interested boys.
"Let me show you," said the Doctor. "Make a torch, for it is growing dark, and come with me down the hill a little way. First look where the needle stands now."