Does the weight of the air vary at different times? Yes; the air does vary in weight at different times. How can we tell when the air changes in weight? By means of a _Barometer_. What is a barometer? It is an instrument having a tube filled with quicksilver. When the air is heavier, how does it affect the quicksilver? The quicksilver rises in the tube. When the air is lighter, how does it affect the quicksilver? The quicksilver sinks in the tube. What is the use of the barometer? It shows the changes about to take place in the weather. When the quicksilver rises, what kind of weather is indicated? It indicates fair weather. When the quicksilver sinks, what kind of weather is indicated? It indicates cloudy or stormy weather. Where is the barometer most useful? It is most useful at sea, to warn the sailors of coming storms. What use is it to farmers? They can use it as a guide when planting and gathering their crops, and thus be better prepared for stormy weather. For what else may the barometer be used? It may be used to tell the height or depth of any place. Does the air become lighter as we ascend? The air does become lighter as we ascend. How does this affect the barometer? As the air becomes lighter, the quicksilver in the barometer sinks lower. At what height does the quicksilver stand at the level of the sea? It stands at the height of thirty inches. When we ascend a mountain 10,000 feet high, how much does the quicksilver sink? It sinks ten inches, and, therefore, stands at the height of only twenty inches. Does the same thing take place when we ascend in a balloon? Yes; the higher the balloon ascends, the lower the quicksilver in the barometer sinks. When we descend into the mines dug in the earth, does the air become heavier? It does; and the quicksilver in the barometer rises. At what temperature does water boil? Water boils at a temperature of _two hundred and twelve_ degrees. When the weight of the air is taken off, will the water boil at a lower temperature? Water will then boil at a lower temperature. Where will water boil at the lowest temperature, on a mountain, or in a valley? It will boil at the lowest temperature on a mountain. At how much lower temperature will water boil, as we ascend? Water will boil at one degree less for every five hundred feet we ascend. How high is the city of Quito, in South America? Quito is nine thousand feet above the level of the sea. At what temperature does water boil there? Water boils, in Quito, at a temperature of one hundred and ninety-four degrees. Will an egg boil hard at Quito? No; because the water passes off into steam at one hundred and ninety-four degrees, and it requires more than one hundred and ninety-four degrees of heat to boil an egg hard. Do persons, while ascending mountains, often have difficulty in cooking their food?