The instruments used in observing the weather are the aneroid and cistern barometers, wet and dry bulb thermometers, wind vane and compass, anemometer and anemograph, and the rainfall. Of all these the barometer is probably the most important. The standard form of the instrument is a tube thirty-four inches long, closed at the top, exhausted of air, and immersed at the bottom in a cup of mercury. The purpose of the barometer is to measure the pressure of the atmosphere. In general, the mercury will stand high in the tube when the weather is fair, and low when it is foul. By noting the minute changes, measured on a graduated scale beside the tube, the observer reads the indications of the barometer. The words "fair," "change," etc., engraved on the front of the instrument are disregarded. They have no significance whatever. The rising or falling of the mercury in the tube is caused by the beginning of those atmospheric changes which precede a storm but are not discernible by our senses. The barometer discerns them for us, and gives warning of weather changes. Of course there are many different conditions which affect the instrument, and the weather observers are instructed in these matters. The aneroid barometer is round, like one of the cheap nickel-plated clocks that are so numerous, and the changes are indicated by a hand moving across a scale on the dial. The weight of the atmosphere is measured not by a column of mercury in a tube, but by the expansion and compression of a small metal box from which the air has been exhausted.