What is a barometer? If we should take a glass tube closed at one end, the area of the cross section of which is one inch square, and fill it with mercury, and while thus filled plunge the open end into a vessel of mercury, it will be found that the amount of mercury remaining in the tube above the level of the mercury in the vessel will weigh about fifteen pounds, if the experiment has been performed at sea-level. This will vary, however, according to the temperature of the air. Of course barometers are tested when the air is at a certain temperature. If the weight of mercury in the tube is fifteen pounds, since it is sustained by the air pressing down on the mercury in the open vessel, it shows that the air-pressure on that open vessel is equal to fifteen pounds to the square inch. In practice, of course, the tubes are made very much smaller. If the air changes so that it is lighter than normal the mercury will fall in the tube, because the pressure on the mercury in the open vessel is less than fifteen pounds to the square inch. And, again, conditions may arise that will condense the air and make it for the time being weigh more than fifteen pounds to the square inch, in which case the mercury will rise in the tube. Thus it will be seen that the barometer will register the slightest change in air pressure. Let us dwell for a moment on the causes of what are commonly called "changes of weather," when we will again revert to the use of the barometer. The use of the telegraph in connection with the establishment of a weather bureau having stations for observation at convenient points throughout the country has contributed much to the science of meteorology. It is found that there are areas of high and low pressure existing at the same time in different parts of the country. These usually have their origin in the far northwest, and follow each other, sweeping down the eastern side of the Rocky Mountains and gradually bending easterly and from that to northeasterly by the time they reach the Atlantic coast. The areas of low pressure are called cyclones, while the areas of high pressure are called anti-cyclones. (By cyclone we do not mean those cloud funnels commonly called by that name that form at certain times of the year in certain sections of the country and produce such destruction of life and property. These storms are usually confined to a narrow strip and are short-lived. They arise undoubtedly from local conditions. A description of these tornadoes--for such is their true name--will be given in some future chapter.)