In order that we may proceed intelligently, let us say a word about the barometer. We speak of high and low barometer, and we make the instrument with graduations marked for all kinds of weather, which really mean but very little. The reading of a single barometer alone will give us but a faint idea of what is really going to happen from day to day. But if we have a series of barometers located at different stations scattered all over the continent and connected at headquarters by telegraph, so that we can have the readings from a whole series of barometers at once, then it becomes a very useful instrument. A barometer may read low at one station by the scale, but may be high with reference to some other barometer that reads very low.
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.